Optical displacement sensor and optical encoder

ABSTRACT

An optical displacement sensor comprises a surface emitting laser light source, a scale and a photosensor. The surface emitting laser light source emits a light beam having a predetermined shape. The scale is displaceable in such a manner as to cross the light beam emitted from the surface emitting laser light source and has a diffraction grating of a predetermined period formed thereon for forming a diffraction interference pattern from the light beam. The photosensor receives a predetermined portion of the diffraction interference pattern. The photosensor includes light intensity detecting means comprised of a plurality of light receiving areas arranged apart from one another in a pitch direction of the diffraction interference pattern on a light receiving surface at intervals of np 1 (z 1 +z 2 )/z 1  where z 1  is a distance between a light-beam emitting surface of the surface emitting laser light source and a surface on which the diffraction grating is formed, z 2  is a distance between the surface on which the diffraction grating is formed and the light receiving surface of the photosensor, p 1  is the pitch of the diffraction grating on the scale, and n is a natural number.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional Application of U.S. applicationSer. No. 09/480,506, which is based upon and claims the benefit ofpriority from the prior Japanese Patent Applications No. 11-006411,filed Jan. 13, 1999, and No. 11-362940, filed Dec. 21, 1999. The entirecontents of parent application Ser. No. 09/480,506 are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical displacement sensor and anoptical encoder and, more particularly, to an optical displacementsensor for detecting the displaced amount of a precision mechanism.

The general structure of an encoder, which is a typical conventionaldisplacement sensor as a first prior art, will be described first. FIG.28 shows the structure of a conventional laser encoder using a coherentlight source and a diffraction grating scale as an example of aminiaturized low cost encoder that does not require assembly of opticalparts such as a lens.

A laser encoder using a coherent light source and a diffraction gratingscale is described in, for example, “Copal: Rotary encoder catalogue”.As shown in FIG. 28, the laser encoder is constructed such that a laserbeam emitted from a semiconductor laser constituting a coherent lightsource 1 is irradiated on a transmission type diffraction grating scale2 to form a diffraction interference pattern 13. A specific portion ofthe diffraction interference pattern 13 thus formed passes throughtransmission slits 53 located at predetermined distances P2 so as to bedetected by a photosensor 3.

FIGS. 29A to 29E show in detail the operation of the displacement sensorusing the laser encoder, which is shown in FIG. 28.

First of all, the operation of the conventional displacement sensorshown in FIG. 28 will be described with reference to FIGS. 29A to 29E.The individual structural parameters shown in FIG. 29A are defined asfollows:

z1: distance between the light source 1 and the surface of the scale 2on which the diffraction grating is formed;

z2: distance between the surface of the scale 2 on which the diffractiongrating is formed and the light receiving surface of the photosensor;

p1: pitch of the diffraction grating on the scale 2;

p2: pitch of a diffraction interference pattern 13 on the lightreceiving surface of the photosensor 3;

θx: spread angle of the light beam emitted from the light source towardthe pitch of the diffraction grating on the scale 2; and

θy: spread angle of the light beam emitted from the light source in adirection perpendicular to the spread angle θx (the spread angle of thelight beam represents an angle made by a pair of boundary lines 6 atwhich the intensity of the light beam becomes a half in a direction inwhich the intensity of the light beam forms a peak).

It should be noted that the “pitch of the diffraction grating on thescale 2” represents the spatial period of a pattern formed by modulatingthe optical characteristics formed on the scale 2. Also, the “pitch ofthe diffraction interference pattern 13 on the light receiving surfaceof the photosensor 3” represents the spatial period of the intensitydistribution of the diffraction interference pattern 13 formed on thelight receiving surface.

According to the diffraction theory of light, an intensity patternsimilar to the diffraction grating pattern of the scale 2 is formed onthe light receiving surface of the photosensor 3, if z1 and z2 definedas above meet the relationship given below:(1/z1)+(1/z2)=λ/kp1²   (1)

where λ is the wavelength of the light beam emitted from the lightsource, and k is an integer.

In this case, the pitch p2 of the diffraction interference pattern 13 onthe light receiving surface can be expressed by an equation (2) givenbelow by using the other structural parameters:p2=p1(z1+z2)/z1   (2)

If the scale 2 is displaced with respect to the light source 1 in thepitch direction of the diffraction grating, the intensity distributionof the diffraction interference pattern 13 is moved in the direction ofdisplacement of the scale while maintaining the same spatial period.

Therefore, if the spatial period p20 of a light receiving area 4 of thephotosensor 3 is set at a value equal to that of p2, a periodicintensity signal is obtained every time the scale 2 is moved by p1 inthe pitch direction, making it possible to detect the amount ofdisplacement of the scale 2 in the pitch direction.

A prior art relating to a small displacement sensor using a surfaceemitting laser light source will now be described as a second prior art.

The small displacement sensor is a complex resonator type interferencesensor using a surface emitting laser light source, which is disclosedin a paper by the present inventors, entitled “Ultra small sensor usinga surface emitting laser” by Eiji Yamamoto, Lecture Materials (VI) forMachinery Institute 75th Periodic General Meeting, 1998, pp. 682-689.

As shown in FIGS. 30A and 30B, this small displacement sensor has asurface emitting laser light source 10 and an outer mirror 61 positionedto face each other so as to constitute a complex resonator. The lightemitted from the surface emitting laser light source 10 is detected bythe light receiving area 4 formed in the photosensor 3 so as to detect achange in the distance L between the surface emitting laser light source10 and the outer mirror 61.

According to this paper, the output characteristics of the sensor whenthe distance L is changed depend on many structural parameters. However,typical examples of calculation are shown in the case of using an edgeemitting type semiconductor laser which is an ordinary conventionalsemiconductor laser widely used (FIG. 31A), in comparison with the useof a surface emitting laser (FIG. 31B).

According to the paper, when an edge emitting type semiconductor laseris used as the light source, the laser output is scarcely changedregardless of a change in the distance L between the light source andthe outer mirror, if the distance L is at least several scores of μm.When a surface emitting laser is used as the light source, by way ofcontrast, a slight change in the distance L greatly changes the laseroutput even if the light source and the outer mirror are positionedapart from each other by over several millimeters.

The paper also teaches that, even in the case of using a surfaceemitting laser as the light source, the laser output is scarcely changedregardless of a change in the distance L, if the distance L is setrelatively large by inclining the outer mirror facing the light source.

FIG. 31C shows the characteristics in the case where the outer mirror isinclined by 0.5° with the structure similar to that shown in FIG. 31B.As also apparent from FIG. 31C, it is known that a change in the laseroutput with respect to a change in the distance L can be suppressed byfurther tilting the outer mirror even if the distance L is small.

Let us consider the case where z1 and z2 deviate from the relationshipgiven by the equation (1) because of an initial misalignment inassembling the sensor and the mechanical swinging caused by thedisplacement of the scale in the prior arts shown in FIGS. 28 and 29A to29E.

For example, where the scale position is deviated by Δz from theposition of the scale 2 to the position of a scale 22 as shown in FIG.29A, though the light source and the light receiving surface are fixed,the diffraction interference pattern on the light receiving surface isdisturbed as shown in FIGS. 29B and 29C as well as the pitch p2 of thediffraction interference pattern 13 on the light receiving surface ischanged according to the equation (2).

It is to be noted that the expression “the diffraction interferencepattern 13 on the light receiving surface is disturbed” implies that thesimilarity of the diffraction interference pattern 13 on the lightreceiving surface to the diffraction grating pattern of the scale 2 isdisturbed.

In the conventional structures shown in FIGS. 28 and 29A to 29E, z1 isz1+Δz and z2 is z2−Δz in the ordinary case where the arrangement of thelight source 1 and the photosensor 3 is fixed.

Let us now consider the case where the scale surface and the lightreceiving surface are arranged in parallel.

If the pitch of the interference pattern formed on the light receivingsurface is changed from p2 to p2′ when a positional deviation Δz hastaken place, the following equation (3) is satisfied:p2′=p1(z1+Δz+z2−Δz)/(z1+Δz)p1(z1+z2)/(z 1+Δz)   (3)

Therefore, if a plurality of light receiving regions 4 of thephotosensor 3 are formed in accordance with the period of the pitch p2,a deviation between the period of the light receiving region and theperiod of the diffraction interference pattern 13 is increased at theposition away from the principal axis of the beam from the light source.As a result, as shown in FIGS. 29D and 29E, it is inevitable that theamplitude of the output signal Ipd from the photosensor is lowered andthe diffraction interference pattern 13 is disturbed.

Suppose that z1=0.5 mm, z2=0.5 mm and

Δz=−z1/10=0.05 mm. In this case, p2=20 μm and p2′=22.2 μm are obtainedfrom an equation (3).

Therefore, even if the pitch of the light receiving area 4 is set atp20=p2=20 μm as designed, 4.5p2=4.0p2′ at the position away from theprincipal axis of the light beam on the light receiving surface bySx/2=4.5p2=90 μm, with the result that the diffraction interferencepattern 13 is deviated by ½ pitch. It follows that a signal 14 outputfrom the light receiving area 4 at this position bears a reversed phase,leading to reduction in the amplitude of the output from the sensor.

In this case, the phase of the diffraction interference pattern 13 isreversed on the light receiving surface at the position where theapparent angle θ when viewed from the light source becomes 2ArcTan(4.5p2/(z1+z2))=10.3°. With this angle taken as the maximumapparent angle θmax, it is desirable to set the distribution width Sx ofthe light receiving area 4 at a value corresponding to about half ofθmax representing the maximum apparent angle and also to set the beamspread angle of the coherent light at about θmax.

The distribution width Sx of the light receiving area noted aboveimplies the entire expansion where the aforementioned plural lightreceiving regions are distributed.

To be more specific, in order to suppress the reduction in the amplitudeof the output signal Ipd caused by the positional deviation of the scaleand a variation in the assembling step of the sensor and to obtain theappropriate light receiving level, it is effective to limit thedistribution width Sx of the light receiving area to an area in thevicinity of the principal axis of the light beam and to use a coherentlight source having a spread angle of the light beam corresponding tothe distribution width.

Under the circumstances, in the case of the conventional structure whichuses an edge emitting type semiconductor laser as the light source, thespread angle of the light beam is very large, i.e., about 40° along thelonger axis and about 20° along the shorter axis, making it difficult toemit a laser beam having a beam spread angle θmax of about 10°. Whatshould be noted is that most of the laser beam spread by an angle largerthan the θmax of about 10° causes a significant reduction in theamplitude of the sensor output or leads to a lower light receivinglevel.

Such being the situation, what is required is a sensor using a coherentlight as the light source that permits the spread angle of the laserbeam emitted from the light source to be set appropriately.

Further, even if the light receiving area is limited to a region in thevicinity of the principal axis of the light beam, it is unavoidable forthe period of the output signal Ipd to change, leading to an error inthe measurement of the absolute value in the amount of displacement ofthe scale.

The requirement to suppress the change in the period of the outputsignal Ipd when a positional deviation Δz (Δz means a gap variationbetween the scale and the head) has taken place in the scale is thestructure that does not bring about a change in the period of thediffraction interference pattern on the light receiving surface.

Another problem inherent to the prior art is that the light emitted fromthe laser light source is reflected on the surfaces of the scale and thephotosensor so as to return to the laser light source, which results ina change in the light intensity occurs, leading to noise generation inthe output signal.

A measure against the difficulty is essential particularly in a casewhere a laser having a small beam spread angle such as a surfaceemitting laser is used as the light source, as already described inconjunction with the second prior art. For suppressing the difficulty,it is necessary to provide the structure that can lower the noise causedby the returning laser light beam.

The following is the summary of the shortcomings of the above-describedprior arts and the subject matters of this invention to cope with theshortcomings.

As the prior arts use a conventional semiconductor laser as the lightsource, the spread angle of the light beam is very large, i.e., about40° in the direction of the longer axis and about 20° in the directionof the shorter axis. In addition, it is impossible to set the spreadangle of the light beam as desired. Therefore, if the expansion of thelight receiving area is limited to a region in the vicinity of theprincipal axis of the light beam, the power of the light incident to thelight receiving area is remarkably lowered, resulting in failure tosolve the problem that the S/N ratio of the signal is lowered.

A first subject matter of this invention, which has been achieved inview of the situation described above, is to provide the structure thatpermits setting the spread angle of the light beam to be set to apredetermined small angle that cannot be achieved by the conventionalsemiconductor laser light source and to realize an optical displacementsensor that provides an output signal of a good S/N ratio even if thearrangement of the light source, the scale and the light receivingelement deviates from the optimum arrangement.

In the prior arts, the surface of the scale and the light receivingsurface of the photosensor are arranged perpendicular to the principalaxis of the light beam emitted from the laser light source. As a result,the light emitted from the laser light source is reflected on thesurfaces of the scale and the photosensor so as to return to the laserlight source, thus generating noise.

A second subject matter of this invention, which has been achieved inview of this situation, is to provide an optical displacement sensorthat prevents the laser light from returning to the light source to bethereby able to suppress the superimposition of the noise caused by thereturning laser light on the output signal of the sensor.

Further, in the case where the arrangement of the light source, thescale and the light receiving element deviates from the designed layoutin the prior art, the period and the position of the diffractioninterference pattern on the light receiving surface are greatly changed,resulting in a failure to suppress the reduction in the amplitude of thesignal and the change in the period with respect to the scaledisplacement.

A third subject matter of this invention, which has been achieved inview of this situation, is to provide an optical displacement sensorcapable of reducing changes in the period and position of thediffraction interference pattern on the light receiving surface and alsocapable of suppressing the reduction of the signal amplitude and changein the period with respect to the scale displacement.

Furthermore, this invention is directed to a fourth subject matter inaddition to the first to third subject matters described previously.

Specifically, the fourth subject matter of this invention is to providean encoder that has the reference point detecting function and theabsolute point detecting function and also can accurately detect thedisplacement of the scale in the x direction while being scarcelyaffected by the change in the gap between the scale and the head.

A fifth subject matter of this invention is to reduce the assemblingcost by employing a mounting mode that does not use an inclinedsubstrate.

Further, a sixth subject matter of this invention is to provide astructure and means that stabilize the encoder output signal in spite ofa change in the environment.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention, which is particularlyintended to achieve the first to third subject matters described above,to provide an optical displacement sensor, which permits the spreadangle of the light beam to be set at an angle not larger than apredetermined value that cannot be achieved in the conventionalsemiconductor laser light source, which prevents laser light capable ofproducing an output signal of a good S/N ratio from returning to thelight source even when the arrangement of the light source, the scaleand the light receiving element is deviated from the optimum arrangement(when the gap between the light source and the scale is varied, forexample) so as to suppress the superimposition of noise caused by thereturning laser light on the output signal of the sensor, and whichreduces changes in the period and position of the diffractioninterference pattern on the light receiving surface so as to suppressreduction in the signal amplitude and a change in the period withrespect to the scale displacement.

It is another object of this invention, which is particularly intendedto achieve the fourth to sixth subject matters described above, toprovide an optical encoder that can be used as an optical displacementsensor, which has a reference point detecting function and an absolutepoint detecting function so as to accurately detect the displacement ofthe scale in an x direction while being hardly affected by the change inthe gap between the scale and the head, and which has a structure andmeans for reducing the assembling cost and for stabilizing the encodersignal output regardless of a change in the environment by employing amounting mode that does not use an inclined substrate.

To achieve the first subject matter, according to a first aspect (1) ofthis invention, there is provided an optical displacement sensorcomprising:

a surface emitting laser light source for emitting a light beam having apredetermined shape;

a scale displaceable in such a manner as to cross the light beam emittedfrom the surface emitting laser light source and having a diffractiongrating of a predetermined period formed thereon for forming adiffraction interference pattern from the light beam; and

a photosensor for receiving a predetermined portion of the diffractioninterference pattern, the photosensor including light intensitydetecting means comprised of a plurality of light receiving areasarranged apart from one another in a pitch direction of the diffractioninterference pattern on a light receiving surface at intervals ofnp1(z1+z2)/z1 where

z1 is a distance between a light-beam emitting surface of the surfaceemitting laser light source and a surface on which the diffractiongrating is formed;

z2 is a distance between the surface on which the diffraction grating isformed and the light receiving surface of the photosensor;

p1 is a pitch of the diffraction grating on the scale; and

n is a natural number.

(Corresponding Embodiment of Invention)

The first aspect (1) of the invention corresponds to a first embodimentof this invention.

While a vertical resonance type surface emitting laser is describedmainly in this embodiment as the surface emitting laser used in thefirst aspect (1) of the invention, the surface emitting laser shouldinclude a surface emitting laser formed by integrating an edge emittingtype semiconductor laser and an optical waveguide, a rising mirror or adiffraction grating.

The “diffraction grating of a predetermined period for forming adiffraction interference pattern” in this structure means a diffractiongrating having a periodic modulated pattern of optical characteristicssuch as the amplitude and phase formed thereon and should cover everydiffraction grating including a reflection type diffraction grating forforming a diffraction interference pattern on the light receivingsurface and a transmission type diffraction grating.

Also, the “light intensity detecting means comprised of a plurality oflight receiving areas” used herein implies a photosensor so constructedas to add the outputs of the plural light receiving areas arranged atintervals of np1(z1+z2)/z1 in the pitch direction of the diffractioninterference pattern on the light receiving surface and output the sumand should include a special case where the light intensity detectingmeans has only one light receiving area.

The value of n that determines the distance between the individual lightreceiving areas should not necessarily be constant over the entireregion.

Further, the displacement sensor performs its function even if thedistance between the individual light receiving areas is slightlydeviated from the numerical condition “np1(z1+z2)/z1” included in theexpression “the plural light receiving areas arranged at intervals ofnp1(z1+z2)/z1 in the pitch direction of the diffraction interferencepattern”. Even if there is a deviation of about ±30% from the numericalcondition “np1(z1+z2)/z1” in working the first aspect (1) of theinvention, therefore, the sensor should be construed to be encompassedwithin the scope of the first aspect (1) of the invention.

(Operation)

According to the optical displacement sensor (1) of the first aspect ofthe invention, the diffraction grating on the scale causes the laserlight emitted from the surface emitting laser to form a diffractioninterference pattern having a predetermined period p1(z1+z2)/z1 on thelight receiving surface of the photosensor.

It should be noted that, since the light receiving areas constitutingthe light intensity detecting means of the photosensor are formed apartfrom one another in the pitch direction of the diffraction grating atintervals of np1(z1+z2)/z1, each light receiving area detects only aspecific portion of the same phase in the diffraction interferencepattern on the light receiving surface.

When the scale is displaced by x1 in the pitch direction of thediffraction grating, the scale is displaced by x2=x1(z1+z2)/z1 in thesame direction on the light receiving surface in the diffractioninterference pattern. Therefore, an output signal changing with aperiodic intensity is acquired from the light intensity detection meansevery time the scale is moved by 1 pitch in the pitch direction of thediffraction grating.

The function of the surface emitting laser light source will now bedescribed more specifically.

FIG. 2A is a cross sectional view schematically showing a surfaceemitting laser, and FIG. 2B is a top view as viewed from a z directionshown in FIG. 2A.

The surface emitting laser shown in FIG. 2A is constructed as describedbelow. Specifically, formed on an N—GaAs substrate 42 is a lamination ofan n-AlGaAs/GaAs semiconductor multi-layer mirror 43, an n-AlGaAs spacerlayer 44, an InGaAs quantum well active layer 45, a p-AlGaAs spacerlayer 46, and a p-AlGaAs/GaAs semiconductor multi-layer mirror 47disposed in the named order. Further, a semi-insulating GaAs currentblocking layer 48 is buried in a region other than the laser resonatorfrom the surface to the n-AlGaAs spacer layer 44 in depth.

The resonator has a diameter ωa and the window through which laser lightemerges has a diameter ωw. The surface shown in FIG. 2A is defined as anxz surface, and the surface shown in FIG. 2B is defined as an xysurface. Further, the spread sizes of the light beam on the lightemitting surface of the surface emitting layer are defined as ωox andωoy in the x and y directions, respectively.

For the sake of descriptive simplicity, the pitch direction of thediffraction grating on the scale is assumed to be the x direction in thedescription of the function in this specification. Also, the surfaceemitting laser emits a light beam having a principal axis 5, and thebeam boundary at which the light intensity becomes half the intensity onthe principal axis is represented by a curve 6.

As shown in FIG. 2A, the angles that are formed by tangential lines 6′tangential to the aforementioned beam boundary curve 6 at a distance andrelative to the principal axis of the light beam are respectively calledspread angles θx and θy of the light beam in the x direction and ydirection.

For a surface emitting laser, for example, in the case of FIG. 2A, asthe sizes ωox and ωoy are changed by freely setting the dimensions ofthe light emerging window of the device, θx and θy can be set in a widerange for the diffraction of the light beam.

FIG. 3 shows the experimental results that have been obtained byevaluating the relationship between the beam size on the light emittingsurface of the surface emitting laser and the beam spread angle θ of aprototype prepared, and shows that setting the beam spread angle in awide range is possible.

It is apparent that the beam size of 3 μm or greater leads to small beamspreading and is a good condition.

The beam size ωo on the light emitting surface can be roughly consideredas either the diameter ωa of the resonator or the diameter of the lightemerging window ωw, whichever is smaller.

When ωa>ωw as shown in FIG. 2A, therefore, the beam size ωo can be takenapproximately as ωw.

When ωa<ωw for another surface emitting laser as shown in FIG. 4, thebeam size ωo can be taken approximately as ωa.

According to the first aspect (1) of the invention, therefore, even ifthe spread of the light receiving area in the x direction is limited toan area in the vicinity of the principal axis of the light beam from thelight source by using the surface emitting laser 10 as the light sourceinstead of the edge emitting type semiconductor laser 1 which has beenused in the conventional encoders, it is possible to set the beam spreadangle corresponding to the narrow spreading of the light receiving areaby properly setting ωox of the surface emitting laser. This can allow adiffraction interference pattern to be formed in the light receivingarea by effectively using the light beam emitted from the light source.

Accordingly, the optical displacement sensor provided can output anoutput signal with an excellent signal amplitude and an excellent S/Nratio even if the arrangement of the light source, the scale and thelight receiving element is deviated from the optimal one.

Because light need not have a diffraction interference in a directionperpendicular to the pitch of the diffraction grating in order tominiaturize the sensor, as small a beam spread angle as possible isdesirable.

To prevent the output signal from being affected by a defect or dust inthe diffraction grating on the scale, on the other hand, it is desirableto set the beam spread angle as large as possible.

To achieve the second subject matter, according to a second aspect (2)of this invention, the optical displacement sensor as described in thesection (1) is characterized in that the photosensor has second lightintensity detecting means having an output terminal independent fromthat of the light intensity detecting means, and

the second light intensity detecting means has a light receiving widthof mp1(z1+z2)/z1 in the pitch direction of the diffraction interferencepattern on the light receiving surface where m is a second naturalnumber which can be set independently of the natural number n.

(Corresponding Embodiment of Invention)

The second aspect (2) of the invention corresponds to second to fourthembodiments of this invention.

Further, the displacement sensor performs its function even if thedistance between the individual light receiving areas is slightlydeviated from the numerical condition “mp1(z1+z2)/z1” included in theexpression “the second light intensity detecting means has a lightreceiving width of mp1(z1+z2)/z1 in the pitch direction of thediffraction interference pattern”. Even if there is a deviation of about±30% from the numerical condition “mp1(z1+z2)/z1” in working the secondaspect (2) of the invention, therefore, the sensor should be construedto be encompassed within the scope of the second aspect (2) of theinvention.

(Operation)

In addition to the feature of the first aspect (1) of the invention, thesecond aspect (2) of the invention has a feature such that thediffraction interference pattern on the light receiving surface has aperiod of p1(z1+z2)/z1, so that forming the second light intensitydetecting means having a light receiving width of mp1(z1+z2)/z1 on thelight receiving surface allows the photosensor to receive thediffraction interference pattern of a period of m. As a result, thephotosensor detects the diffraction interference pattern on the lightreceiving surface at an average intensity level.

Since the average intensity level on the light receiving surface isproportional to the light beam output from the laser light source, thesecond light intensity detecting means is equivalent to an additionalfunction of monitoring the light beam output from the laser lightsource.

Even if the ambient environment of the sensor or the status of thereturning light changes, therefore, the optical output is stabilized byfeeding the output of the second light intensity detecting means to thedrive means for the laser light source, thus ensuring stable sensing tosuch a status change.

To achieve the third subject matter, according to a third aspect (3) ofthis invention, there is provided an optical displacement sensorcomprising:

a light source for emitting coherent light;

a scale displaceable in such a manner as to cross a light beam as thecoherent light emitted from the light source and having a diffractiongrating of a predetermined period for forming a diffraction interferencepattern from the light beam; and

a photosensor for receiving a predetermined portion of the diffractioninterference pattern,

whereby a principal axis of the light beam as the coherent light emittedfrom the light source is tilted in a predetermined direction to a lineperpendicular to that surface of the scale on which the light beam isirradiated.

(Corresponding Embodiment of Invention)

The third aspect (3) of the invention corresponds to a seventhembodiment of this invention.

According to the third aspect (3) of the invention, the scale surface orthe light receiving surface of the photosensor is tilted to theprincipal axis of the light beam emitted from the laser light source toensure variable sensing by the scale at a higher precision and higherreliability. This arrangement can prevent the light emitted from thelaser light source and reflected at the surface of the scale or thephotosensor from returning to the laser, thereby suppressingsuperimposition of noise carried by the returning laser light on theoutput signal of the sensor.

To achieve the fourth subject matter, according to a fourth aspect (4)of this invention, there is provided an optical encoder comprising:

a coherent light source;

a scale movably supported and formed with a first scale pattern and asecond scale pattern for reflecting or diffracting and passing a lightbeam from the coherent light source;

a beam-splitting optical element, provided between the coherent lightsource and the scale, for splitting the light beam emitted from thecoherent light source into a plurality of beams;

first and second photosensors for detecting the light beams split by thebeam-splitting optical element,

the first photosensor having a plurality of light receiving areas formedat intervals of approximately np11(z11+z21)/z11 in a spatial perioddirection of a diffraction interference pattern formed on a lightreceiving surface as a first light beam split by the beam-splittingoptical element is optically affected the first scale pattern, where z11is an optical distance along a principal axis of the first light beamfrom a beam emitting surface of the coherent light source to a surfacewhere the first scale pattern is formed, z21 is an optical distancealong the principal axis of the first light beam to the firstphotosensor from the surface where the first scale pattern is formed tothe first photosensor, p11 is a spatial period of the first scalepattern and n is a natural number,

a second light beam among the plurality of light beams split by thebeam-splitting optical element being optically affected the second scalepattern and being then received by the second photosensor.

(Corresponding Embodiment of Invention)

The fourth aspect of the invention corresponds to thirteenth tonineteenth embodiments of this invention.

While a vertical resonance type surface emitting laser 10 is describedmainly in those embodiments as the “coherent light source”, this lightsource should include an ordinary semiconductor laser 1 and other laserlight sources which can emit a coherent light beam.

The term “optical distance” means the distance measured with referenceto the wavelength of the light beam.

When the refractive index is constant over the area where the distanceis to be measured, for example, the “optical distance” is ageometrically measured distance, whereas when the refractive indexvaries, the “optical distance” means a change in optical wavelength bythe refractive index distribution reflected on the geometric distancealong the path to be measured.

The “scale pattern of a predetermined period for forming a diffractioninterference pattern” means a diffraction grating having a periodmodulated pattern of optical characteristics such as the amplitude andphase formed thereon and should cover every diffraction gratingincluding a reflection type diffraction grating for forming adiffraction interference pattern on the light receiving surface and atransmission type diffraction grating.

Also, the “photosensor has a plurality of light receiving areas formedat intervals of approximately np11(z11+z21)/z11 in the spatial perioddirection of the diffraction interference pattern and receives apredetermined portion of the diffraction interference pattern” implies agroup of light receiving elements so constructed as to add the outputsof the plural light receiving areas arranged at intervals ofnp11(z11+z21)/z11 in the pitch direction of the diffraction interferencepattern on the light receiving surface and output the sum and shouldinclude a case where there are plural groups of light receivingelements.

The value of n that determines the distance between the individual lightreceiving areas should not necessarily be constant over the entireregion.

Further, the displacement sensor performs its function even if thedistance between the individual light receiving areas is slightlydeviated from the numerical condition “np11(z11+z21)/z11” included inthe expression “the plural light receiving areas arranged at intervalsof np11(z11+z21)/z11 in the pitch direction of the diffractioninterference pattern”. Even if there is a deviation of about ±30% fromthe numerical condition “np11(z11+z21)/z11” in working this aspect ofthe invention, therefore, the optical encoder should be considered to beencompassed within the scope of this aspect of the invention.

The term “optically affected” includes at least one of “reflected”,“diffracted” and “transmitted”.

(Operation)

The light beam emitted from the coherent light source is split into aplurality of light beams by the beam-splitting optical element, and thefirst light beam among those light beams is irradiated on the firstscale pattern, thus forming a diffraction interference pattern having apredetermined period p11(z11+z21)/z11 on the light receiving surface ofthe first photosensor.

The following description will be given of a case of n=1 for the sake ofsimplicity.

It should be noted that, since the light receiving areas of the firstphotosensor are formed apart from one another in the pitch direction ofthe diffraction grating at intervals of p21=np2=np11(z11+z21)/z11, eachlight receiving area detects only a specific portion of the same phasein the diffraction interference pattern on the light receiving surface.

When the scale is displaced by x1 in the pitch direction of thediffraction grating, the scale is displaced by x2=x1(z11+z21)/z11 in thesame direction on the light receiving surface in the diffractioninterference pattern. Therefore, an output signal which changes with aperiodic intensity is acquired from the first photosensor every time thescale is moved by p11 in the pitch direction of the diffraction grating.

The second light beam among those light beams split by thebeam-splitting optical element is optically affected the second scalepattern and its intensity is detected by the second photosensor.

Combining the first and second scale patterns as will be discussed latercan provide an intensity monitoring function, absolute positiondetecting function and a reference point (or origin) detecting functionfor the optical output of the coherent light source.

Two or more of those additional functions may be achieved simultaneouslyby increasing the number of split beams and the number of thephotosensors in accordance with the number of the scale patterns.

As the coherent light source, the scale surface and the light receivingsurface can all be arranged in parallel to one another, a mounting modewhich does not involve an inclined substrate is possible, which can leadto a significant reduction in assembling cost.

To achieve the fifth subject matter, according to a fifth aspect (5) ofthis invention, the optical encoder as described in the section (4) ischaracterized by further comprising:

a first optical beam-bending element provided between the scale and thefirst photosensor; and

a second optical beam-bending element provided between the scale and thesecond photosensor,

whereby the first and second light beams which have been opticallyaffected the first scale pattern and the second scale pattern passthrough the first and second optical beam-bending elements to bereceived by the first and second photosensors, respectively.

(Corresponding Embodiment of Invention)

This aspect of the invention corresponds to sixteenth to eighteenthembodiments of this invention.

According to this structure, the first optical beam-bending element andthe second optical beam-bending element may be integrated with thebeam-splitting optical element or may be separate therefrom.

(Operation and Effect)

This section does not discuss the same function and effect as those ofthe fourth aspect (4) of the invention.

After the first and second light beams which have been split by thefirst beam-splitting optical element are respectively irradiated on thefirst and second scale patterns and then irradiated on the first andsecond optical beam-bending elements where the optical axes of the beamsare bended again so that the beams are respectively led to the first andsecond photosensors.

If the angles by which the first beam-splitting optical element and thefirst and second optical beam-bending elements deflect the optical axesof the associated beams are set equal to one another, the principal axesof the first and second light beams become symmetrical to a lineextending perpendicularly from a point where the principal axes crossthe scale surface.

As has been mentioned in the section of the prior arts, it is apparentfrom the equation (2) that even if the spatial gap between the scale 2and the light source varies, arranging the light source and thephotosensor on the same side with respect to the scale 2 and settingz11=z21 and z21=z22 prevent the pitch of the diffraction interferencepattern on the light receiving surface from changing.

Therefore, this structure of the invention has an advantage such thatthe optical distance from the light source to the scale surface, whichhas been measured along the principal axes of the first and second lightbeams, to the light receiving element can be made equal to the opticaldistance from the scale to the light receiving element with a verysimple design.

If the height of the light receiving surface and the imaginary positionof the light source differ from each other, an optical element forcompensating for the difference has only to be inserted in the opticalpath of the optical axis.

To achieve the sixth subject matter, according to a sixth aspect (6) ofthis invention, there is provided an optical encoder comprising:

a coherent light source;

a scale movably supported and formed with a first scale pattern and asecond scale pattern for reflecting or diffracting and passing a lightbeam from the coherent light source;

a beam-splitting optical element, provided between the coherent lightsource and the scale, for splitting the light beam emitted from thecoherent light source into a plurality of beams;

first and second photosensors for detecting the light beams split by thebeam-splitting optical element, the first photosensor having a pluralityof light receiving areas formed at intervals of approximatelynp11(z11+z21)/z11 in a spatial period direction of a diffractioninterference pattern formed on a light receiving surface as a firstlight beam split by the beam-splitting optical element is opticallyaffected the first scale pattern, where z11 is an optical distance alonga principal axis of the first light beam from a beam emitting surface ofthe coherent light source to a surface where the first scale pattern isformed, z21 is an optical distance along the principal axis of the firstlight beam to the first photosensor from the surface where the firstscale pattern is formed to the first photosensor, p11 is a spatialperiod of the first scale pattern and n is a natural number,

a second light beam among the plurality of light beams split by thebeam-splitting optical element being received by the second photosensorwithout being irradiated on any scale pattern.

(Corresponding Embodiment of Invention)

This aspect of the invention corresponds to a nineteenth embodiment ofthis invention.

(Operation and Effect)

This section does not discuss the same functions and effects as those ofthe fourth and fifth aspects (4) and (5) of the invention.

Of the light beams split by the beam-splitting optical element, thesecond light beam is directly detected by the second photosensor withoutbeing irradiated on the scale.

This structure can thus achieve a function for monitoring the opticaloutput of the coherent light source. Even if the ambient environment ofthe optical encoder which can be adapted as an optical displacementsensor varies, therefore, the optical output can be stabilized byfeeding the output of the light intensity detecting means which iscomprised of the second photosensor to the drive means for the laserlight source. This can ensure stable sensing to such a status change.

As this structure does not require a scale having a uniform reflectanceor transmissivity, it leads to a reduction in the cost of the scale andis not affected by a defect or dust or the like in the scale as comparedwith the embodiments to be discussed later.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIGS. 1A to 1C, which illustrate the schematic structure of an opticaldisplacement sensor according to a first embodiment of this invention,are respectively a perspective view showing the first embodiment, across-sectional view of an xz plane corresponding to FIG. 1A and across-sectional view of a yz plane corresponding to in FIG. 1A;

FIGS. 2A and 2B, which exemplify a surface emitting laser in FIGS. 1A to1C, are respectively a schematic cross-sectional view of the surfaceemitting laser and a top view as viewed from a −z direction in FIG. 2A;

FIG. 3 is a diagram showing the experimental results that have beenobtained by evaluating the relationship between the beam size on thelight emitting surface of the surface emitting laser and the beam spreadangle θ of a prototype prepared;

FIG. 4 is a schematic cross-sectional view showing another example ofthe surface emitting laser in FIGS. 1A to 1C;

FIGS. 5A and 5B are diagrams showing an example where a reflectionpreventing film 36(a) for suppressing reflection and a treating section37(b) which provides a light scattering effect are formed on a scale 2and the light receiving surface of a photosensor 3 by respective opticalprocesses in order to reduce noise in the returning light from the scaleand the light receiving surface in FIGS. 1A to 1C;

FIG. 6A is a perspective view showing an optical displacement sensoraccording to a second embodiment;

FIG. 6B is a plan view showing the light receiving surface of aphotosensor 3 in FIG. 6A as viewed towards the −z direction;

FIG. 7 is a plan view illustrating an optical displacement sensoraccording to a third embodiment and, like FIG. 6B, showing the lightreceiving surface of the photosensor 3 in FIG. 6A as viewed from thescale side;

FIG. 8 is a diagram exemplifying a structure in which two separategroups of light receiving areas in FIG. 7 are alternately formed by anodd multiple of p2/4;

FIG. 9 is a plan view illustrating an optical displacement sensoraccording to a fourth embodiment and, like FIG. 6B, showing the lightreceiving surface of the photosensor 3 in FIG. 6A as viewed towards the−z direction;

FIG. 10 is a perspective view showing an optical displacement sensoraccording to a fifth embodiment;

FIG. 11 is a perspective view showing an optical displacement sensoraccording to a sixth embodiment;

FIGS. 12A and 12B, which illustrate the schematic structure of anoptical displacement sensor according to a seventh embodiment, arerespectively a cross-sectional view of an xz plane of the opticaldisplacement sensor according to the seventh embodiment and across-sectional view of a yz plane corresponding to FIG. 12A;

FIGS. 13A and 13B are diagrams showing the results of computing adiffraction interference pattern on the light receiving surface with atilt angle φ=0 under a predetermined condition in comparison with thecomputation results in a case where the tilt angle φ=10°;

FIG. 14 is a diagram depicting a case of using an optical part (e.g., aprism) which deflects the optical axis as a modification of a scheme oftilting the light beam emitted from a light source to the scale surface;

FIGS. 15A and 15B, which illustrate the schematic structure of anoptical displacement sensor according to an eighth embodiment, arerespectively a cross-sectional view of an xz plane of the opticaldisplacement sensor according to the eighth embodiment and across-sectional view of a yz plane corresponding to FIG. 15A;

FIGS. 16A and 16B, which illustrate the schematic structure of anoptical displacement sensor according to a ninth embodiment, arerespectively a cross-sectional view of an xz plane of the opticaldisplacement sensor according to the ninth embodiment and across-sectional view of a yz plane corresponding to FIG. 16A;

FIGS. 17A and 17B, which illustrate the schematic structure of anoptical displacement sensor according to a tenth embodiment, arerespectively a cross-sectional view of an xz plane of the opticaldisplacement sensor according to the tenth embodiment and across-sectional view of a yz plane corresponding to FIG. 17A;

FIGS. 18A and 18B, which illustrate the schematic structure of anoptical displacement sensor according to an eleventh embodiment, arerespectively a cross-sectional view of an xz plane of the opticaldisplacement sensor according to the eleventh embodiment and across-sectional view of a yz plane corresponding to FIG. 18A;

FIGS. 19A to 19C, which illustrate the schematic structure of an opticaldisplacement sensor according to a twelfth embodiment of this invention,are respectively a perspective view showing the twelfth embodiment, across-sectional view of an xz plane corresponding to FIG. 19A and across-sectional view of a yz plane corresponding to in FIG. 19A;

FIGS. 20A and 20B, which illustrate the structure of an opticaldisplacement sensor to be used as an optical encoder according to athirteenth embodiment of this invention, are respectively a plan viewshowing the scale 2 in the thirteenth embodiment as viewed towards a −zdirection and a cross-sectional view of a yz plane in this embodiment;

FIGS. 21A to 21C, which illustrate the structure of an opticaldisplacement sensor to be used as the optical encoder according to thethirteenth embodiment of this invention, are respectively a plan viewshowing the scale 2 in the thirteenth embodiment as viewed towards a −zdirection, a cross-sectional view of an xz plane corresponding to thecross section along a1-a1′ in FIG. 21A and a cross-sectional view of anxz plane corresponding to the cross section along a2-a2′ in FIG. 21A;

FIGS. 22A to 22C, which illustrate the structure of an opticaldisplacement sensor to be used as an optical encoder according to afourteenth embodiment of this invention, are respectively a plan viewshowing the scale 2 in the fourteenth embodiment as viewed from a −zdirection, a cross-sectional view of an xz plane corresponding to thecross section along a1-a1′ in FIG. 22A and a cross-sectional view of anxz plane corresponding to the cross section along a2-a2′ in FIG. 22A;

FIGS. 23A to 23C, which illustrate the structure of an opticaldisplacement sensor to be used as an optical encoder according to afifteenth embodiment of this invention, are respectively a plan viewshowing the scale 2 in the fifteenth embodiment as viewed from a −zdirection, a cross-sectional view of an xz plane corresponding to thecross section along a1-a1′ in FIG. 23A and a cross-sectional view of anxz plane corresponding to the cross section along a2-a2′ in FIG. 23A;

FIG. 24A is a plan view showing the scale 2 in a sixteenth embodiment asviewed from a −z direction;

FIG. 24B is a plan view showing the scale 2 in a seventeenth embodimentas viewed from a −z direction;

FIG. 24C is a plan view showing the scale 2 in an eighteenth embodimentas viewed from a −z direction;

FIG. 24D is a cross-sectional view illustrating the structure of theoptical displacement sensor which is to be used as the optical encodersaccording to the sixteenth to eighteenth embodiments of this invention;

FIGS. 25A to 25C, which illustrate the structure of an opticaldisplacement sensor to be used as an optical encoder according to anineteenth embodiment of this invention, are respectively a plan viewshowing the scale 2 in the nineteenth embodiment as viewed from a −zdirection, a cross-sectional view of a zy plane and a cross-sectionalview of a xz plane;

FIG. 26 is a perspective view illustrating the structure of an opticaldisplacement sensor to be used as an optical encoder according to atwentieth embodiment of this invention;

FIG. 27 is a perspective view illustrating the structure of an opticaldisplacement sensor to be used as an optical encoder according to atwenty-first embodiment of this invention;

FIG. 28 is a structural diagram showing a laser encoder, as the firstprior art, using a coherent light source and a diffraction grating scaleas an example of a miniaturized low cost encoder that does not requireassembly of optical parts such as a lens;

FIGS. 29A to 29E are operational diagrams for more specificallyexplaining the displacement sensor using the laser encoder in FIG. 28;

FIGS. 30A and 30B are structural diagrams for explaining a prior artrelating to a small displacement sensor using a surface emitting laserlight source as the second prior art;

FIGS. 31A to 31C are diagrams showing a case where an edge emitting typesemiconductor laser which is an ordinary conventional semiconductorlaser widely used is used as the light source (FIG. 31A), in comparisonwith a case where a surface emitting laser is used (FIG. 31B) and thecharacteristics when an outer mirror is tilted by 0.5° (FIG. 31C) astypical examples of calculation for explaining that the outputcharacteristics of the sensor when the distance L is changed depend onmany structural parameters; and

FIGS. 32A and 32B are diagrams for explaining the simplest modes in thecase of expanding the structure of shown in FIGS. 16A and 16B into themulti-beam & track configuration.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the presently preferredembodiments of the invention as illustrated in the accompanyingdrawings, in which like reference numerals designate like orcorresponding parts.

Preferred embodiments of this invention will be described below withreference to the accompanying drawings.

FIRST EMBODIMENT

FIGS. 1A to 1C illustrate the schematic structure of an opticaldisplacement sensor according to a first embodiment of this invention.FIG. 1A is a perspective view showing the structure of the firstembodiment, FIG. 1B is a cross-sectional view of an xz planecorresponding to FIG. 1A and FIG. 1C is a cross-sectional view of a yzplane corresponding to in FIG. 1A.

As shown in FIGS. 1A to 1C, a surface emitting laser light source 10 anda scale 2 are arranged with such a positional relationship that thelight beam emitted from this light source 10 is irradiated on the scale2, and a photosensor 3 is located so as to receive a predeterminedportion of a diffraction interference pattern formed from the lightsource by the scale 2.

An area 4 in FIGS. 1A to 1C indicates a light receiving area where thephotosensor 3 is so located as to receive a predetermined portion of thediffraction interference pattern. If a plurality of light receivingareas 4 are provided, the light receiving areas 4 are mutually connectedby electric wirings 31 so that the sensor output can be acquired from anoutput pad 32.

A dotted line 5 in FIGS. 1A to 1C indicates the principal axis of thelight beam emitted from the surface emitting laser light source 10, andsolid lines 6 indicate boundary lines for the spreading of the lightbeam. An area 15 in FIGS. 1A to 1C show the light-beam spreading area onthe surface where the diffraction grating of the scale 2 is formed andan area 16 in FIGS. 1A to 1C shows the light-beam spreading area on thelight receiving area of the photosensor 3.

To secure the clearness of the diffraction interference pattern on thelight receiving surface, as mentioned earlier, it is desirable that thedistances among the surface emitting laser light source 10, the scale 2and the photosensor 3 should satisfy the relationship given by theequation (1).

The operation of the first embodiment of this invention will now bedescribed.

The light beam emitted from the surface emitting laser light source 10which has a predetermined beam shape is irradiated on the scale 2 wherethe diffraction grating is formed.

The scale 2 is so displaced as to cross the light beam from the surfaceemitting laser 10.

The light that has been diffracted by the scale 2 generates adiffraction interference pattern on the light receiving surface of thephotosensor 3. A single or plural of light receiving area 4 formed onthe photosensor 3 detects a predetermined portion of the diffractioninterference pattern.

As the sensor signal periodically changes in accordance with thedisplacement of the scale 2 across the light beam, the amount ofdisplacement of the scale 2 can be detected.

To acquire a signal with a higher S/N ratio by increasing the outputsignal of the sensor, the light receiving area 4 on the photosensor 3 isformed by integrating a plurality of areas in such a way as to have agiven spatial period p20 in the same direction as the pitch direction ofthe diffraction grating of the scale 2, as shown in FIGS. 1A to 1C.

Because it is desirable for the spatial period p20 of the lightreceiving areas to be equal to the period p2 of the diffractioninterference pattern on the light receiving surface, p20 is setapproximately equal to np1(z1+z2)/z1.

According to the first embodiment, the surface emitting laser lightsource 10 is used as the light source instead of the conventionalsemiconductor laser 1 in FIG. 28. Even if the area 16 where the lightreceiving area is formed is restricted to an area close to the principalaxis of the light beam from the light source 10, therefore, it ispossible to set the beam spread angle corresponding to the spreading ofthe area 16 for the formation of the light receiving area. This allowsthe diffraction interference pattern to be produced in the area 16 forthe formation of the light receiving area by effectively using theamount of light from the light source 10.

By not spreading the area where the diffraction interference pattern isto be formed more than needed, the pitch deviation between thediffraction interference pattern on the light receiving area and thedistributed light receiving area 16 can be reduced even when thearrangement of the surface emitting laser light source 10, the scale 2and the photosensor 3 is shifted from the optimal one. Accordingly, thisoptical displacement sensor is capable of providing an output signalwhich has an excellent signal amplitude and excellent S/N ratio.

The spread angle of the light beam from the surface emitting laser lightsource 10 can be designed freely with respect to the scale pitchdirection (x direction) and a direction perpendicular to the scale pitch(y direction), for example, the light-beam spreading width over aplurality of pitches can be set on the surface of the scale 2 in thepitch direction of the scale 2, and a narrower light-beam spreadingwidth can be set in the direction perpendicular to the former direction.It is therefore possible to minimize the sizes of the scale 2 and thephotosensor 3, which leads to miniaturization of the sensor and costreduction.

The individual components of the first embodiment of this invention canof course be changed or modified in various forms.

Although a vertical resonance type surface emitting laser has beendiscussed mainly in this embodiment as the surface emitting laser lightsource 10, for example, the surface emitting laser should include asurface emitting laser formed by integrating an edge emitting typesemiconductor laser and an optical waveguide, a rising mirror or adiffraction grating should be covered.

The “diffraction grating of a predetermined period for forming adiffraction interference pattern” in this structure means a diffractiongrating having a period modulated pattern of optical characteristicssuch as the amplitude and phase formed thereon and should cover everydiffraction grating including a reflection type diffraction grating forforming a diffraction interference pattern on the light receivingsurface and a transmission type diffraction grating.

As the light source is the surface emitting laser light source 10, whenthe surface of the photosensor 3 and the principal axis of the lightbeam are set perpendicular to each other, noise on the returning lightfrom the scale 2 or the light receiving surface is large as shown inFIGS. 31A to 31C.

To reduce this noise, it is desirable to perform an optical treatment onthe scale 2 and the light receiving surface of the photosensor 3 tosuppress reflection.

For example, it is desirable to form a reflection preventing film 36 onthe scale 2 and the light receiving surface of the photosensor 3 asshown in FIG. 5A, or to form a treating section 37 which has fineupheavals on its surface to provide a light scattering effect as shownin FIG. 5B.

It is to be noted that this optical treatment to suppress reflection canlikewise be applied to the other embodiments of this invention, thoughthis description will not be repeated to avoid the redundantdescription.

SECOND EMBODIMENT

An optical displacement sensor according to a second embodiment of thisinvention will now be described with reference to FIGS. 6A and 6B.

FIG. 6A is a perspective view showing the optical displacement sensoraccording to the second embodiment, and FIG. 6B is a plan view showingthe light receiving surface of the photosensor 3 in FIG. 6A as viewedtowards the −z direction.

For those portions which are common to those of the first embodiment,their description will partly be omitted below.

In addition to the light intensity detecting means that is comprised ofa plurality of light receiving areas formed at intervals ofp2=np1(z1+z2)/z1, the aforementioned second light intensity detectingmeans is formed with a light receiving width of pm=mp1(z1+z2)/z1 (wherem is a natural number different from n) in the pitch direction of thediffraction interference pattern on the light receiving surface, andthose light intensity detecting means are connected to an outputacquisition pad 32 and a pad 36 via the wirings 31 in the diagrams.

The area 16 in FIGS. 16A and 16B indicates the light-beam spreading areaon the light receiving surface. It is desirable that the aforementionedplurality of light receiving areas or the light receiving area of thesecond light intensity detecting means should be formed in this area 16.

The operation of the second embodiment of this invention will now bedescribed.

An electric signal is periodically output from the pad 32 every time thescale 2 is displaced by P1 in the pitch direction. An electric signalwhich is proportional to the laser output from the light source 10 isoutput from the pad 36.

The electric signal output from the pad 36 serves to monitor the laseroutput from the light source 10. As this electric signal is fed back tothe means (not shown) for driving the light source 10, a variation inthe laser output can be suppressed, for example, when the ambienttemperature or pressure changes or the returning light changes the laseroutput.

This can ensure stable displacement sensing even if the environmentaround the sensor changes.

Note that the integration of the second light intensity detecting meansin the photosensor can also be applied to the other embodiments of thisinvention.

THIRD EMBODIMENT

An optical displacement sensor according to a third embodiment of thisinvention will now be described with reference to FIG. 7.

FIG. 7 is a plan view of the light receiving surface of the photosensor3 in FIG. 6A as viewed towards the −z direction.

For those portions which are common to those of the second embodiment,their description will be partly omitted below.

There are two sets of light intensity detecting means which are eachcomprised of a plurality of light receiving areas formed at intervals ofp2=np1(z1+z2)/z1 and are connected to output pads 32 and 33 via thewirings 31.

The two groups of light receiving areas are shifted from each other byδp20 in the x direction on the light receiving surface.

The operation of the third embodiment of this invention will now bedescribed.

When the scale 2 is displaced in the pitch direction, electric signalsof different phases are output from the output pads 32 and 33.

Using the phase relationship between the electric signals output fromthe output pads 32 and 33, it is possible to detect the direction of themovement of the scale 2 or a minute amount of displacement equal to orless than the pitch p1 caused by the phase segmentation of the signals.

Referring to FIG. 7, if the spatial layout deviation δp20 of pluralgroups of light receiving areas is set to an odd multiple of p2/4 in thepitch direction of the scale 2, the phase difference of the signals fromeach group of light receiving areas is shifted by a ¼ period or ¾period, thus yielding so-called encoder signals of phase A and phase B.

The two groups of light receiving areas in FIG. 7 may be formed in sucha fashion that different groups of light receiving areas are alternatelyformed and deviated by an odd multiple of p2/4 from each other.

In this case, even if the light beam is shifted in the y direction fromthe optimal state, the ratio of the average output level from one groupof light receiving areas to that of the other group hardly changes, thusensuring stable sensing.

FOURTH EMBODIMENT

An optical displacement sensor according to a fourth embodiment of thisinvention will now be described with reference to FIG. 9.

FIG. 9 is a plan view of the light receiving surface of the photosensor3 in FIG. 6A as viewed from the scale side.

With regard to those portions which are common to those of the secondand third embodiments, their description will be partly omitted below.

According to the fourth embodiment, four groups of light receiving areasare alternately formed from one another by δp20.

Although wirings 31 and 31′ in FIG. 9 are so drawn as to cross eachother, they are formed into a multi-layer structure by respectiveinterconnection layers or the like and are thus electrically isolatedfrom each other.

Referring to FIG. 9, “32”, “33”, “34” and “35” denote pads for acquiringelectric signals from the individual groups of light receiving areas.

The spatial layout deviation δp20 of plural groups of light receivingareas is normally set to an odd multiple of p2/4.

The operation of the fourth embodiment of this invention will now bediscussed.

The pads 32, 33, 34 and 35 output four signal signals, respectively,each having a phase difference of a quarter (¼) cycle with respect tothe immediate preceding signal. More precisely, these signals are the Aphase, B phase, A phase and B phase of a so-called “encoder signal.”

The A-phase signal and the A-phase signal have the opposite phases, andthe B-phase signal and the B-phase signal have the opposite phases.Hence, if a signal representing the phase difference between the A-phaseand A-phase signals and a signal representing the phase differencebetween the B-phase and the B-phase signals are used, it is possibleperform stable signal detection, not influenced by stray light comingfrom the environment.

In this invention, the intensity of the laser beam can be monitored bycalculating the sum of the outputs A phase, B phase, A-phase, B-phase,making it possible to correct to some extent the influence of a changein environment and a time-dependent change on the change in intensity ofthe laser beam by feeding back the sum to make constant the change inintensity of the laser beam caused by the change in environment and bytime-dependent change, or by performing an adequate arithmetic operationon the output signal of the phase A (or phase B) and the signal of thesum of the individual outputs the A, B, A and B phases.

FIFTH EMBODIMENT

An optical displacement sensor according to a fifth embodiment of thisinvention will now be described with reference to FIG. 10.

The description of those portions which are common to those of the firstembodiment will be partly omitted below.

The surface emitting laser light source 10 emits two light beams fromdifferent positions, which are both irradiated on the scale 2.

Areas 15 and 15′ in FIG. 10 indicate the light-beam spreading areas onthe scale 2, and diffraction interference patterns are so formed as tolie over those areas.

The diffraction interference patterns corresponding to the areas 15 and15′ are chiefly formed in portions 16 and 16′ in the diagram, and groupsof light receiving area are formed so as to detect a predeterminedspatial phase portion of each diffraction interference pattern.

It is desirable that the amount of deviation δp20 of those groups oflight receiving areas in the pitch direction should be set to an oddmultiple of p2/4.

The operation of the fifth embodiment of this invention will now bediscussed.

With the structure as shown in FIG. 10, the output pads 32 and 33provide so-called phase-A and phase-B outputs.

This embodiment can be modified in various forms.

For example, the second light intensity detecting means may beintegrated to monitor the light beam or four groups of light receivingareas may be provided, yielding four-phase outputs of phase A, phase B,phase A and phase B, as illustrated in the foregoing sections of thesecond to fourth embodiments.

SIXTH EMBODIMENT

An optical displacement sensor according to a sixth embodiment of thisinvention will now be discussed with reference to FIG. 11.

The description of those portions which are common to those of the firstembodiment will be partly omitted below.

Two groups of diffraction grating patterns are formed on the scale 2 andare arranged in such a way that the light beams emitted from the surfaceemitting laser light source 10 are respectively irradiated on bothdiffraction grating patterns.

The two groups of diffraction gratings generate diffraction interferencepatterns on different areas on the light receiving surface of thephotosensor 3 and two groups of light receiving areas formed on thephotosensor 3 are so formed as to selectively receive only specificphase portions of those diffraction interference patterns.

It is desirable that the positional deviation of the two groups ofdiffraction gratings on the scale 2 in the pitch direction should be setto an odd multiple of p2/4.

The operation of the sixth embodiment of this invention will now bediscussed.

By setting the positional deviation between the two groups ofdiffraction gratings on the scale 2 in the pitch direction to an oddmultiple of p1/4, diffraction interference patterns of spatial phasesdifferent from each other by p2/4 in the pitch direction are formed onthe light receiving surface of the photosensor 3 (p2: the pitch of thediffraction interference patterns on the light receiving surface).

Even when the two groups of light receiving areas formed on thephotosensor 3 are set without a positional deviation in the pitchdirection, therefore, signals having a phase difference of an oddmultiple of the ¼ period are acquired from the pads 32 and 33.

It is thus possible to detect the direction of the movement of the scale2 or a minute amount of displacement equal to or less than the pitch p1caused by the phase segmentation of the signals, as per the secondembodiment.

With this structure, as the distance between the emission positions ofthe two light beams from the surface emitting laser light source 10becomes larger, the diffraction interference patterns corresponding tothose positions can be formed spatially apart from each other. This cansuppress such a problem that the diffraction interference pattern in thebeam spreading area 15 on the scale 2 is slightly detected by the groupof light receiving areas 4′ which should receive the other diffractioninterference pattern, thus reducing the interference of the signals tobe output from the pads 32 and 33.

This embodiment can be modified in various forms.

For example, the second light intensity detecting means may beintegrated to monitor the light beam or four groups of light receivingareas may be provided, yielding four-phase outputs of phase A, phase B,phase A and phase B, as illustrated in the foregoing sections of thesecond to fourth embodiments.

SEVENTH EMBODIMENT

An optical displacement sensor according to a seventh embodiment of thisinvention will now be discussed with reference to FIGS. 12A and 12B.

The description of those portions which are common to those of the firstto fifth embodiments will be partly omitted below.

With a structure similar to that of the first embodiment, the principalaxis of the light beam emitted from the surface emitting laser lightsource 10 is tilted by a predetermined angle φ to a line perpendicularto the surface of the scale 2.

It is desirable as will be discussed later that the surface of the scale2 should be set in parallel to the light receiving surface of thephotosensor 3.

The operation of the seventh embodiment of this invention will now bediscussed.

Tilting the principal axis of the light beam by the predetermined angleφ to the scale 2 reduces the light returning to the surface emittinglaser light source 10 from the scale 2 so that even when the opticaldistance between the scale 2 and the light source 10 is changedslightly, the output of the light source 10 can be kept stable.

As one example, FIG. 31B shows the results of computation of a variationin the output of the light source for φ=0 in a case where the lightsource is a surface emitting laser and the distance between the scale 2or the mirror that is regarded as the light receiving surface and thebeam emitting surface of the surface emitting laser or LD is changed,and FIG. 31C shows the results of such computation for φ=0.5°. (Theresults of the computation are recited in the aforementioned paper.)

The computation results show that even slight tilting of 0.5°demonstrate a good effect of reducing a variation in the output of thelight source and further tilting can suppress a variation in the outputof the light source caused by the returning light even if there is asmall distance between the light source and the mirror (equivalent tothe scale in this invention).

When the light source is a surface emitting laser as mentioned above,particularly, this structure demonstrates a superb effect of reducingnoise generated by the returning light.

When the scale surface is arranged in parallel to the light receivingsurface, provided that L1 and L2 are the mutual distances of the lightsource, the scale surface and the light receiving surface on the linesthat connect the light source to the scale surface and the lightreceiving surface and z1 and z2 are the mutual distances between thelight source and the scale surface and the light receiving surface onlines perpendicular to the scale, then z1/z2=L1/L2 every where on thelight receiving surface as expected from the equation (2), so it ispredicted that the pitch of the diffraction interference pattern on thelight receiving surface becomes constant.

When the tilt angle is small, the equation (1) appears to beapproximately met in the beam spreading area Sx by making z1 and z2satisfy the equation (2). It is therefore expected that the clearness ofthe diffraction interference pattern is hardly changed at almosteverywhere in the area Sx from that of the case of φ=0.

To check those points, the results of computing a diffractioninterference pattern on the light receiving surface with a tilt angleφ=0 under the following conditions and the computation results in a casewhere the tilt angle φ=10° are respectively illustrated in FIGS. 13A and13B in comparison with each other.

(Computation Conditions)

optical wavelength λ=1 μm

distance between the light source and scale z1=500 μm distance betweenthe light receiving surface and scale z2=500 pm beam spread angle θx=7°(on the assumption that the intensity distribution in the x direction isrectangular)

the scale surface is arranged in parallel to the light receiving surface

It is apparent from FIGS. 13A and 13B that even if the scale is tilted,arranging the scale surface in parallel to the light receiving surfacedoes not change the pitch of the diffraction interference pattern in thex direction, and that the tilt angle of about 10° results in just slightreductions in the periodicity and clearness of the diffractioninterference pattern on the light receiving surface.

Even if the scale surface is tilted to the principal axis of the laserbeam, that is, the scale surface and the light receiving surface are inparallel each other, with p1, z1 and z2 being fixed values, the spatialperiod of the diffraction interference pattern in the x directionbecomes constant so that the pitch of the light receiving areas has onlyto be constant with respect to the x direction. This facilitates thedesign of the light receiving areas.

Because of the same reason, there is another advantage such that evenslight deviation of arrangement of the photosensor in the pitchdirection of the scale does not raise a problem of a deviation in pitchbetween the diffraction interference pattern and the light receivingareas.

It is to be noted that when the distance z1 between the scale and thelight source is changed by Δz, the phase (or the peak position) of theoptical displacement sensor diffraction interference pattern on thelight receiving surface is shifted in the x direction in the positionalrelationship illustrated in FIGS. 12A and 12B.

This embodiment can be modified in various forms.

The structures and arrangement of the light source, the scale and thephotosensor in this embodiment can be modified as those of the second tosixth embodiments, thereby additionally providing the above-describedeffects associated with those embodiments.

The scheme of tilting the light beam emitted from the surface emittinglaser light source with respect to the scale surface may be implementedby using optical parts (e.g., a prism) which deflects the optical axisas shown in FIG. 14 as well as tilting the direction of the light sourceas shown in FIGS. 12A and 12B.

EIGHTH EMBODIMENT

An optical displacement sensor according to an eighth embodiment of thisinvention will now be discussed with reference to FIGS. 15A and 15B.

The description of those portions which are common to those of theseventh embodiment will be partly omitted below.

Although the seventh embodiment has not defined the structure where theprincipal axis of the light beam from the light source is setperpendicular to a specific direction of the scale surface, the pitchdirection of the diffraction grating formed on the scale is arrangedperpendicular to the principal axis of the light beam which is emittedfrom the light source 1, 10 that emits the coherent light and thesurface where the diffraction grating of the scale 2 is formed isarranged in parallel to the light receiving surface of the photosensoraccording to the eighth embodiment.

According to the eighth embodiment, therefore, a direction perpendicularto the pitch direction of the diffraction grating formed on the scale 2and the principal axis of the light beam emitted from the light sourcethat emits coherent light are tilted by an angle φ.

The operation of the eighth embodiment of this invention will now bediscussed.

According to this embodiment, as the principal axis of the light beam isset perpendicular to the pitch direction of the diffraction grating,even if the distance between the scale 2 and the light source 1, 10changes by Δz, the diffraction interference pattern to be produced onthe light receiving surface becomes symmetrical to the principal axis ofthe light beam so that a deviation of the peak position of thediffraction interference pattern on the light receiving surface becomessmall in the vicinity of the principal axis of the light beam.

The movement of the diffraction interference pattern in the x directioncaused by a variation of Δz which may raise a problem in the sixthembodiment is suppressed in the vicinity of the principal axis of thelight beam, thus ensuring precise sensing of the displacement of thescale 2 in the x direction.

The use of the surface emitting laser which has a beam spread widthcorresponding to the distribution of the light receiving areas in thevicinity of the principal axis of the light beam can permit theeffective use of the intensity of the laser light, thereby ensuringsensing with a high S/N ratio.

This embodiment can be modified in various forms.

The structures and arrangement of the light source, the scale and thephotosensor in this embodiment can be modified as those of the second tosixth embodiments, thereby additionally providing the above-describedeffects associated with those embodiments.

The scheme of tilting the light beam emitted from the surface emittinglaser light source with respect to the scale surface may be implementedby using optical parts (e.g., a prism) which deflects the optical axisas shown in FIG. 14 as well as tilting the direction of the light sourceas shown in FIG. 15.

NINTH EMBODIMENT

An optical displacement sensor according to a ninth embodiment of thisinvention will now be discussed with reference to FIGS. 16A and 16B.

The description of those portions which are common to those of the sixthembodiment will be partly omitted below.

The light beam emitted from the light source 1, 10 is diffracted on thescale 2, and is returned to the same side as the light source 1, 10 withrespect to the scale 2, thereby generating a diffraction interferencepattern on the light receiving surface of the photosensor 3 which islocated on the same side as the light source 1, 10.

A light reception pattern is formed on the photosensor 3 in such a waythat the photosensor 3 receives a predetermined portion of thisdiffraction interference pattern.

In the example in FIGS. 16A and 16B, the photosensor 3 is arranged atthe position equivalent to the position of a photosensor 33 folded withrespect to the scale surface to cope with the possible case where thelight beam emitted from the light source 1, 10 is diffracted on thescale 2 and passes through the opposite side to the light source 1, 10with respect to the scale 2.

Reference numeral “11” in FIGS. 16A and 16B denotes a block forintegrating and securing the light source 1, 10 and the photosensor 3.

The operation of the ninth embodiment of this invention will now bediscussed.

The photosensor 3 outputs a periodic output every time the scale 2 isdisplaced by p1 in the x direction as indicated by an output curve 13 inFIGS. 16A and 16B.

Although the pitch of the diffraction interference pattern on the lightreceiving surface slightly changes as given by the equation (3) when thedistance z1 between the scale 2 and the light source 1, 10 is increasedby Δz as illustrated, this structure has an advantage of reducing achange in the pitch of the diffraction interference pattern as apparentfrom the following equation (4) as compared with the case where thephotosensor 3 is arranged on the opposite side to the light source withrespect to the scale 2. $\begin{matrix}\begin{matrix}{{p\quad 2^{\prime}} = {p\quad 1{( {{z\quad 1} + {\Delta\quad z} + {z\quad 2} + {\Delta\quad z}} )/( {{z\quad 12} + {\Delta\quad z}} )}}} \\{= {p\quad 1{( {{z\quad 1} + {z\quad 2} + {2\Delta\quad z}} )/( {{z\quad 1} + {\Delta\quad z}} )}}}\end{matrix} & (4)\end{matrix}$

Under the condition of z1=z2, particularly, even if there has occurred achange Δz in the distance between the scale 2 and the light source 1,10, the pitch of the diffraction interference pattern on the lightreceiving surface does not deviate as apparent from the equation (3). Itis thus desirable that the arrangement should be made under thecondition of z1=z2.

It is to be noted however that this structure still has such a problemthat when the distance z1 between the scale 2 and the light source 1, 10is increased by Δz as illustrated, the position of the diffractioninterference pattern on the light receiving surface deviates, so thatthe output curve 13 of the sensor is shifted by Δxb as indicated by anoutput curve 14.

This embodiment can be modified in various forms.

The structures and arrangement of the light source, the scale and thephotosensor in this embodiment can be modified as those of the second tosixth embodiments, thereby additionally providing the above-describedeffects associated with those embodiments.

The scheme of tilting the light beam emitted from the surface emittinglaser light source with respect to the scale surface may be implementedby using optical parts (e.g., a prism) which deflects the optical axisas shown in FIG. 14 as well as tilting the direction of the light sourceas shown in FIGS. 16A and 16B.

TENTH EMBODIMENT

An optical displacement sensor according to a tenth embodiment of thisinvention will now be discussed with reference to FIGS. 17A and 17B.

The description of those portions which are common to those of the ninthembodiment will be partly omitted below.

Although the ninth embodiment has not defined the structure where theprincipal axis of the light beam from the light source is setperpendicular to a specific direction of the scale surface, the pitchdirection of the diffraction grating formed on the scale is arrangedperpendicular to the principal axis of the light beam which is emittedfrom the light source 1, 10 that emits the coherent light and thesurface where the diffraction grating of the scale 2 is formed isarranged in parallel to the light receiving surface of the photosensoraccording to the tenth embodiment.

Therefore, a direction perpendicular to the pitch direction of thediffraction grating formed on the scale 2 and the principal axis of thelight beam emitted from the light source that emits coherent light aretilted by an angle φ.

The operation of the tenth embodiment of this invention will now bediscussed.

According to this embodiment, as the principal axis of the light beam isset perpendicular to the pitch direction of the diffraction grating,even if the distance between the scale 2 and the light source 1, 10changes by Δz, so that a deviation of the peak position of thediffraction interference pattern on the light receiving surface canmostly be neglected by restricting the distribution of the lightreceiving areas to the vicinity of the principal axis of the light beambecause the diffraction interference pattern is symmetrical on the xzplane to the principal axis of the light beam as shown in FIG. 17A.

This brings about such an advantage that a variation of Δz, if occurredat the time of sensing, hardly affect the detection of the amount ofmovement in the x diffraction.

The use of the surface emitting laser which has a beam spread widthcorresponding to the distribution of the light receiving areas in thevicinity of the principal axis of the light beam can permit theeffective use of the intensity of the laser light, thereby ensuringsensing with a high S/N ratio.

Because the principal axis of the light beam and the directionperpendicular to the pitch direction of the diffraction grating areinclined and the structure is of a reflection type in the ninthembodiment, like the eighth embodiment, this embodiment has advantagessuch that even if a deviation of Δz occurs at the time of sensing, avariation in the output of the light source caused by the lightreturning from the scale 2 or the photosensor 3 is small and a change inthe pitch of the diffraction interference pattern on the light receivingsurface is small.

Particularly, setting z1 equal to z2 can ensure stable sensing such thateven if a deviation of Δz occurs at the time of sensing, the pitch andthe peak position of the diffraction interference pattern hardly change.

This embodiment can be modified in various forms.

The structures and arrangement of the light source, the scale and thephotosensor in this embodiment can be modified as those of the second tosixth embodiments, thereby additionally providing the above-describedeffects associated with those embodiments.

The scheme of tilting the light beam emitted from the surface emittinglaser light source with respect to the scale surface may be implementedby using optical parts (e.g., a prism) which deflects the optical axisas shown in FIG. 14 as well as tilting the direction of the light sourceas shown in FIGS. 17A and 17B.

ELEVENTH EMBODIMENT

An optical displacement sensor according to an eleventh embodiment ofthis invention will now be discussed with reference to FIGS. 18A and18B.

The description of those portions which are common to those of the tenthembodiment will be partly omitted below.

According to this embodiment, the pitch direction of the diffractiongrating formed on the scale is arranged perpendicular to the principalaxis of the light beam which is emitted from the light source 1, 10 thatemits the coherent light and the surface where the diffraction gratingof the scale 2 is formed is arranged in parallel to the light receivingsurface of the photosensor 3.

The operation of the eleventh embodiment of this invention will now bediscussed.

Because of the reflection type structure, this embodiment, like thetenth embodiment, has advantages such that even if the distance betweenthe scale 2 and the light source 1, 10 is changed, a variation in thepitch of the diffraction interference pattern becomes smaller and thedeviation of the peak position of the diffraction interference patternbecomes smaller in the vicinity of the principal axis of the light beam,as compared with the transmission type structure used in the prior arts.

Particularly, the use of the surface emitting laser which has a beamspread width corresponding to the distribution of the light receivingareas in the vicinity of the principal axis of the light beam can permitthe effective use of the intensity of the laser light, thereby ensuringsensing with a high S/N ratio.

According to the embodiment, as φ=0, it should be noted that returnlight noise is generated by the light that returns to the light sourcefrom the scale or the light receiving surface when z1 is small. In theusage where such noise generation is negligible, however, thisembodiment has an advantage such that the structure becomes simpler.

Even if z1 is small, the noise that is caused by the light returning tothe light source from the scale or the light receiving surface isreduced by subjecting the top and bottom surfaces of the scale or thelight receiving surface to an optical process of reducing thereflection, this structure can be adapted for any use where the returnlight noise is negligible.

Particularly, setting z1 equal to z2 can ensure stable sensing such thateven if a deviation of Δz occurs at the time of sensing, the pitch andthe peak position of the diffraction interference pattern hardly change.

TWELFTH EMBODIMENT

An optical displacement sensor according to a twelfth embodiment of thisinvention will now be discussed with reference to FIGS. 19A to 19C.

The description of those portions which are common to those of the firstembodiment will be partly omitted below.

This twelfth embodiment is a special case of the first embodiment wherethere is a single light receiving area, and is the same as the firstembodiment otherwise.

With regard to the ninth embodiment described previously from adifferent aspect relating to the structure of an optical encoder thatcan be used as an optical displacement sensor will now be described.

FIGS. 16A and 16B show the structure of the optical encoder according tothe ninth embodiment from a different point of view.

As shown in FIGS. 16A and 16B, the optical encoder is constructed insuch a way that a laser beam emitted from a semiconductor laser 1 (or asurface emitting laser 10) which is a coherent light source isirradiated on a transmission type diffraction grating scale 2, forming adiffraction interference pattern, and a specific portion of thediffraction interference pattern is detected by either the photosensor 3or a photosensor 33.

In the following description, the coherent light source may simply becalled a light source in some cases.

Where the coherent light source of the semiconductor laser 1 and thephotosensor 3 are arranged on the same side with respect to the scale 2,a principal axis 5 of the light beam emitted from the semiconductorlaser 1 (or the surface emitting laser 10) is inclined by an angle φ tothe line perpendicular to the surface of the scale 2, as shown in FIG.16A.

The operation of the optical encoder of this particular structure whichcan be used as an optical displacement sensor will now be described.

As shown in FIG. 16A, the individual structural parameters are definedas follows as described previously:

z1: distance between the light source and the surface of the scale 2having a diffraction grating formed thereon measured along the principalaxis of the light beam;

z2: distance between the surface of the scale 2 having a diffractiongrating formed thereon and the light receiving surface of thephotosensor 3 measured along the principal axis of the light beam;

p1: pitch of the diffraction grating formed on the scale 2;

p2: pitch of the diffraction interference pattern on the light receivingsurface of the photosensor 3;

θx: spread angle of the light beam of the light source with respect tothe pitch direction of the diffraction grating on the scale 2;

θy: spread angle of the light beam of the light source in a directionperpendicular to θx given above.

The spread angle of the light beam represents an angle made by a pair ofboundary lines 6 at which the intensity of the light beam becomes a halfin a direction in which the intensity forms a peak.

It should be noted that the “pitch of the diffraction grating on thescale 2” represents the spatial period of a pattern formed by modulatingthe optical characteristics formed on the scale 2. Also, the “pitch ofthe diffraction interference pattern on the light receiving surface ofthe photosensor 3” represents the spatial period of the intensitydistribution of the diffraction interference pattern formed on the lightreceiving surface.

According to the diffraction theory of light, an intensity patternsimilar to the diffraction grating pattern of the scale is formed on thelight receiving surface of the photosensor, if z1 and z2 meet therelationship given by the equation (1).

In this case, the pitch p2 of the diffraction interference pattern onthe light receiving surface can be represented by the equation (2) byusing the other structural parameters.

If the scale 2 is displaced with respect to the light source 1 in thepitch direction of the diffraction grating, the intensity distributionof the diffraction grating pattern is moved in a direction ofdisplacement of the scale while maintaining the same spatial period.

Therefore, if the spatial period p20 of the light receiving area 4 ofthe photosensor 3 is set at a value equal to that of p2, a periodicintensity signal is obtained every time the scale 2 is moved by p1 inthe pitch direction, making it possible to detect the amount ofdisplacement of the scale 2 in the pitch direction.

If the light source 1 and the photosensor 3 are arranged on the sameside relative to the scale 2 in such a way that z1 is equal to z2(z1=z2), as shown in FIGS. 16A and 16B, the pitch of the diffractioninterference pattern on the light receiving surface remain unchanged asapparent from the equation (2) even if the spatial gap between the scale2 and the light source 1 is changed.

Further, in practice, a group of the light receiving elements, eachhaving a spatial period P20, of the photosensor is shifted by a distanceof P20/4 to form four groups of light receiving elements that arearranged alternately. In this case, Va-Va′ and Vb-Vb′, where Va, Vb, Va′and Vb′ represent the outputs of the light receiving elements of thefour groups, respectively, are utilized as so-called “phase A” (sinewave) output and “phase B” (cosine wave) output of the encoder.

In this invention to be described later, the intensity of the laser beamcan be monitored by calculating the sum of the outputs Va, Vb, Va′, Vb′,making it possible to correct to some extent the influence of a changein environment and a time-dependent change on the change in intensity ofthe laser beam by feeding back the sum to make constant the change inintensity of the laser beam caused by the change in environment and bythe time-dependent change, or by performing an adequate arithmeticoperation on the output signal of the phase A (or phase B) and thesignal of the sum of the individual outputs Va, Vb, Va′ and Vb′.

In the ninth embodiment described previously, the principal axis 5 ofthe light beam is held perpendicular to the pattern direction of thescale as shown in FIG. 16B.

Suppose, in the ninth embodiment, the relationship between z1 and z2fails to meet the equation (1) because of the initial arrangement errorin assembling the optical encoder that can be used as an opticaldisplacement sensor and because of the mechanical swinging caused by thedisplacement of the scale.

As one example, suppose that the photosensor 3 arranged on the same sideas the light source with respect to the scale is used.

For example, where the light source and the light receiving surface arefixed and the scale is deviated by Δz from the position of the scale 2to the position of the scale 22, i.e., where the gap between the sensorhead and the scale has been changed, the light intensity distributionshown in FIG. 32A is changed from a curve 13 to curve 14. In otherwords, the output of the encoder is changed not only because thediffraction interference pattern on the light receiving surface isslightly disturbed but also because the position of the diffractioninterference pattern on the light receiving surface is moved in an xdirection.

Therefore, if the displacement of the scale in the x direction ismeasured in the ninth embodiment, an error is generated due to thechange in the gap between the scale and the head.

If each of a plurality of tracks differing from one another in the scalepattern is irradiated with a laser beam, it is possible to realize thereference point detecting function and the absolute position detectingfunction. For the encoder of the ninth embodiment, however, here hasbeen proposed no scheme of providing a plurality of scale patterns so asto add the reference point detecting function and the absolute positiondetecting function.

In order to allow the encoder to perform these functions while avoidingthe influences inherent in the ninth embodiment, which are caused by achange in the gap between the scale and the head, a special structure isrequired as described in the later section in conjunction with the meansfor their solution given by this invention.

The structure that a plurality of tracks differing from one another inthe scale pattern is irradiated with a laser beam is called herein“multi-beam & track configuration” for the sake of simplicity.

The structures shown in FIGS. 32A and 32B are considered to be thesimplest types in the case where the structure of the ninth embodimentdescribed previously is expanded into the multi-beam & trackconfiguration.

FIG. 32A covers the case where one of the multi-tracks forms a singlepattern for detecting the reference point detection.

Likewise, FIG. 32B covers the case where one of the multi-tracks isformed into a diffraction pattern formed at a pitch differing from thepattern of the other tracks. In this case, the absolute value can bemeasured by the principle of vernier encoder within a range of movementof the scale that is the least common multiple of both pitches.

If the displacement of the scale in the x direction is to be measured inthese structures, however, the diffraction interference pattern is movedin the x direction on the light receiving surface under the influencecaused by the change in the gap between the scale and the head, asdescribed previously. As a result, a measuring error in the displacementin the x direction is inevitable.

A second problem to be considered is that it is necessary to arrange inan inclined fashion the laser light emitting surface of the lightsource, the scale surface and the light receiving surface of thephotosensor, making it difficult to assemble the optical encoder used asan optical displacement sensor.

In the actual manufacturing process, it is necessary to carry out diebonding of the inclined substrate surface of the semiconductor laser 1(or surface emitting laser 10) and further wire bonding of wirings tothe inclined semiconductor laser electrodes under observation by amicroscope.

In these assembling steps, a tool for correcting the inclination isrequired. It is also necessary to move the focal point of the microscopeup and down for the microscopic observation of the inclined parts. Thismakes the assembling steps highly difficult, leading to a longerassembling time and a higher assembling cost.

A third problem of the ninth embodiment is that, in practical use of theencoder, swinging is caused in accordance with the feedback of theoutput of the light source and the movement of the scale at the time ofcorrection because the sum of the outputs Va, Vb, Va′, Vb′, which isused in detecting the intensity of the laser light, is slightly changedwhen the scale is moved. The exact reason for the difficulty is unknown,but it seems reasonable to understand that the interference pattern onthe light receiving surface is slightly deviated from the theoreticshape.

THIRTEENTH EMBODIMENT

FIGS. 20A and 20B illustrate the structure of an optical displacementsensor to be used as an optical encoder according to the thirteenthembodiment of this invention. FIGS. 21A to 21C illustrate the structureof an optical displacement sensor to be used as the optical encoderaccording to the thirteenth embodiment of this invention so as toachieve the fourth to sixth subject matters described above.

FIGS. 20A and 21A are plan views showing the scale 2 in the thirteenthembodiment and a modification thereof as viewed towards the −zdirection. FIG. 20B is a cross-sectional view of the yz plane in thisembodiment.

FIG. 21B is a cross-sectional view of the xz plane corresponding to thecross section along a1-a1′ in FIG. 21A showing this embodiment. FIG. 21Cis likewise a cross-sectional view of the xz plane corresponding to thecross section along a2-a2′ in FIG. 21A.

The thirteenth embodiment of this invention is constructed as follows.

As shown in FIG. 20B, the laser beam emitted from the surface emittinglaser 10 as the coherent light source is split into at least twocomponents by a diffraction grating 7 which is used as a beam-splittingoptical element.

In FIG. 20B, the principal axis of the first light beam immediatelyafter being split is indicated by a solid line 5, and the principal axisof the second light beam immediately after being split is indicated by asolid line 5′.

Those first and second light beams are respectively irradiated on thefirst and second scale patterns 21 and 22 on the scale 2 and arereturned along the lines indicated by the principal axes 15 and 15′ ofthe light beams to be respectively received by first photosensors 3 and3′ (this case will hereinafter be called a reflection type structure),or pass through the scale 2 along the principal axes 25 and 25′ of thelight beams to be respectively received by second photosensors 33 and33′ (this case will hereinafter be called a transmission typestructure).

In the reflection type structure of this embodiment, the first scalepattern 21 has a period of p11 in the x direction and has a reflectancewhich changes periodically, while the second scale pattern 22 has auniform reflectance.

In the transmission type structure, the first scale pattern 21 has aperiod of p11 in the x direction and has a transmissivity which changesperiodically, while the second scale pattern 22 has a uniformtransmissivity.

As shown in FIG. 20B, the photosensor 3 is adhered to a casing 11 andthe surface emitting laser 10 is mounted on the surface of thephotosensor 3.

A transparent cover 61 which serves to both seal the opticaldisplacement sensor which is used as an optical encoder according to thethirteenth embodiment and to secure the diffraction grating is locatedon the top surface of the casing 11.

As shown in FIG. 21B, the first photosensor 3 or 33 has groups ofstrip-like light receiving areas with a period of p21 where p21 isapproximately np11(z11+z21)/z11. In this embodiment, n is set equal to1.

A solid line 6 indicates the boundary line defining the spreading of thelight beam.

The operation of the thirteenth embodiment of this invention and itsmodification will now be discussed.

The interference pattern generated by the first light beam produces adiffraction interference pattern with a period p21 on the lightreceiving surface of the first photosensor 3 or 33 in the x direction.

Normally, there are four groups of light receiving areas arranged atdistances of p21/4 as shown in FIG. 21B, and those groups respectivelyoutput signals of phase A, phase B, phase /A and phase /B as so-calledencoder signals.

That is, as the scale 2 is shifted by p11 in the x direction, periodicoutputs whose phases differ from the phase A, phase B, phase /A andphase /B by ¼ period are acquired.

Because the second scale pattern is a uniform optical pattern, theoutput of the second photosensor 3′ or 33′ becomes proportional to thebeam output.

When the beam output varies due to an environmental change or with thepassage of time, the output of the second photosensor 3′ or 33′ is fedback to the driving unit for the coherent light source 1 to suppress avariation in the output or the encoder output signals are compensated byusing the output of the second photosensor 3′ or 33′, thereby alwaysensuring stable sensing of the displacement of the scale 2.

To optimally adjust the optical distance of the principal axis of thelight beam that travels from the light source 1, 10 to the scale 2 andthe optical distance of the principal axis 15, 15′ of the light beamthat travels from the scale 2 to the first photosensor 3 or 33 (thisadjustment will hereinafter be called optical distance adjustment), itis desirable to set the thickness t of the diffraction grating 7.

The individual components of the thirteenth embodiment of this inventioncan of course be changed or modified in various forms.

While the surface emitting laser 10 is exemplified as the coherent lightsource, an ordinary edge emitting type semiconductor laser or othercoherent light sources may be used as well.

Although the diffraction grating 7 is exemplified as the beam-splittingoptical element, another optical element such as a prism or half mirrormay be used as well.

The aforementioned optical distance adjustment is not limited to theadjustment of the thickness of the diffraction grating but an opticalmedium with a different reflectance may be inserted on the optical axisor a part for adjusting the heights of the light source and thephotosensor may be attached.

FOURTEENTH EMBODIMENT

FIGS. 22A to 22C illustrate the structure of an optical displacementsensor to be used as an optical encoder according to the fourteenthembodiment of this invention.

The cross-sectional structure in the yz plane according to thisembodiment is the same as that of FIG. 20B, and the behavior of theprincipal axis of the light beam that travels from the coherent lightsource 1 to the first photosensor 3 is the same as that of thethirteenth embodiment in the optical sense.

The mode and operation for the first light beam to reach the firstphotosensor 3 through the scale 2 and acquiring the encoder signals fromthis photosensor 3 are also the same as those of the thirteenthembodiment.

The following section of this embodiment will therefore discuss only thedifference where the second scale pattern 22 differs from that of thethirteenth embodiment.

FIG. 22A is a plan view showing the scale 2 in the fourteenth embodimentas viewed from the −z direction. FIG. 22B is a cross-sectional view ofthe xz plane corresponding to the cross section along a1-a1′ in FIG.22A. FIG. 22C is a cross-sectional view of the xz plane corresponding tothe cross section along a2-a2′ in FIG. 22A.

The fourteenth embodiment of this invention is constructed as follows.

The pitch p12 of the second scale pattern 22 is made different from thepitch p11 of the first scale pattern 21.

In accordance with this difference, the light receiving areas are formedlike strips in cross section in the xz plane which corresponds to thecross section along a2-a2′ as shown in FIG. 22C, with the pitch beingp22 in association with the pitch of the second scale pattern, andgroups of light receiving areas are deviated from one another by p22/4.

The deviation p22 of the groups of light receiving areas isapproximately p12(z12+Z22)/Z12.

The operation of the fourteenth embodiment of this invention will now bediscussed.

The second light beam forms a diffraction interference pattern on thelight receiving surface of the second photosensor 3′ in the same manneras the first light beam forms a diffraction interference pattern on thelight receiving surface of the first photosensor 3, and the secondphotosensor 3′ provides a period output signal every time the scale 2 isshifted by p12 in the x direction.

Because the intensities of the outputs of the first photosensor 3 andthe second photosensor 3′ change at the respective periods of p11 andp12 in the pitch direction of the scale 2, therefore, the absoluteposition detecting function can be added within the range of themovement of the scale that is the least common multiple of p11 and p12based on the principle of the vernier encode.

The individual components of the fourteenth embodiment of this inventioncan of course be changed or modified in various forms.

For example, the number of light beams to be split may be set to two orgreater and scale patterns and photosensors which correspond in numberto that number should be provided.

FIFTEENTH EMBODIMENT

FIGS. 23A to 23C illustrate the structure of an optical displacementsensor to be used as an optical encoder according to the fifteenthembodiment of this invention.

The cross-sectional structure in the yz plane according to thisembodiment is the same as that of FIG. 20B, and the behavior of theprincipal axis of the light beam that travels from the coherent lightsource 1 to the first photosensor 3 is the same as that of thethirteenth embodiment in the optical sense.

The mode and operation for the first light beam to reach the firstphotosensor 3 through the scale 2 and acquiring the encoder signals fromthis photosensor 3 are also the same as those of the thirteenthembodiment.

The following section of this embodiment will therefore discuss only thedifference where the second scale pattern 22 differs from that of thethirteenth embodiment.

FIG. 23A is a plan view showing the scale 2 in the fifteenth embodimentas viewed from the −z direction. FIG. 23B is a cross-sectional view ofthe xz plane corresponding to the cross section along a1-a1′ in FIG.23A. FIG. 23C is a cross-sectional view of the xz plane corresponding tothe cross section along a2-a2′ in FIG. 23A.

The fifteenth embodiment of this invention is constructed as follows.

The second scale pattern 22 is formed by an optical pattern having aperiod pz at a specific reference position.

In accordance with this difference, the cross-sectional structure in thexz plane which corresponds to the cross section along a2-a2′ is the sameas that of the thirteenth embodiment as shown in FIG. 23C.

The operation of the fifteenth embodiment of this invention will now bediscussed.

When the second light beam is irradiated on the portion at which thereflectance or transmissivity of the second scale pattern 22 changes,the output of the second photosensor 3′ changes. Therefore, thereference point detecting function can be added by previously formingthe portion where the reflectance or transmissivity of the second scalepattern 22 changes at the desired position.

The individual components of the fifteenth embodiment of this inventioncan of course be changed or modified in various forms.

The second scale pattern 22 may has a single pattern or may have aplurality of patterns formed at reference point intervals according tothe usage.

The number of light beams to be split may be set to two or greater andscale patterns and photosensors which correspond in number to thatnumber should be provided.

SIXTEENTH TO EIGHTEENTH EMBODIMENTS

FIGS. 24A to 24D illustrate the structures of optical displacementsensors to be used as an optical encoder according to the sixteenth,seventeenth and eighteenth embodiments of this invention. FIGS. 24A to24C are plan views respectively showing the sixteenth to eighteenthembodiments as viewed from the −z direction.

The sixteenth to eighteenth embodiments have the same cross-sectionalstructure in the yz plane as shown in FIG. 24D.

With regard to the cross-sections in the xz plane, FIGS. 21B and 21C,FIGS. 22B and 22C and FIGS. 23B and 23C respectively correspond to thesixteenth to eighteenth embodiments.

The sixteenth to eighteenth embodiments of this invention areconstructed as follows.

In the sixteenth to eighteenth embodiments, the first scale pattern 21,the second scale pattern 22, the first photosensor 3 or 33 and thesecond photosensor 3′ or 33′ are the same as those of the thirteenth tofifteenth embodiments, respectively.

As shown in FIG. 24D, the laser beam emitted from the surface emittinglaser 10 as the coherent light source is split into at least twocomponents by a diffraction grating 7 which is used as a beam-splittingoptical element.

In FIG. 24D, the principal axis of the first light beam immediatelyafter being split is indicated by a solid line 5, and the principal axisof the second light beam immediately after being split is indicated by asolid line 5′.

Those first and second light beams are respectively irradiated on thefirst and second scale patterns 21 and 22 on the scale 2 and arereturned along the lines indicated by the principal axes 15 and 15′ ofthe light beams, and their optical axes are deflected by diffractiongratings 71 and 72 which are used as first and second opticalbeam-bending elements, so that the resultant light beams arerespectively received by the first photosensors 3 and 3′ (this case willhereinafter be called a reflection type structure), or pass through thescale 2 along the principal axes 25 and 25′ of the light beams and theiroptical axes are deflected by a diffraction grating 77 which is used asthe first and second optical beam-bending elements, so that theresultant light beams are respectively received by the secondphotosensors 33 and 33′ (this case will hereinafter be called atransmission type structure).

The operation of the sixteenth to eighteenth embodiments of thisinvention will now be discussed.

The operations of sixteenth to eighteenth embodiments are the same asthose of the thirteenth, fourteenth and fifteenth embodiments except forthe light beams being deflected by the first and second beam-bendingelements 77.

If the pitches pg of the diffraction gratings 7 and 77 which are used asthe beam-splitting optical element and the first and second opticalbeam-bending elements are set equal to each other, the principal axis ofthe light beam immediately after it has been emitted from the lightsource 1 becomes in parallel to the principal axis of each light beamthat enter the photosensor 3 or the like.

In this case, in FIG. 24D, the principal axis of the light beam thattravels from the light source 1 to the scale 2 becomes symmetrical tothe principal axis of each light beam that travels from the scale 2 tothe photosensor 3 or the like with respect to lines b1-b1′ and b2-b2′perpendicularly extending from the points where the first and secondlight beams intersect the scale 2 to the scale surface. This structurehas such an advantage as to facilitate computation to make z11=z21 andz21=z22 in the design.

This can permit easy designing to set z11=z21 and z21=z22 however thedeflection angles of the principal axes are set by the beam-splittingoptical elements. The sixteenth to eighteenth embodiments therefore areadvantageous in designing the gap between the scale and the sensor, thepitch of the scale and so forth with a large degree of freedom.

Of course, the individual components of the sixteenth to eighteenthembodiments of this invention can be changed or modified in variousforms.

Where the difference between the imaginary position of the light sourceand the height of the light receiving surface raises a problem, opticaldistance adjusting means 50 may be inserted on the optical axis of thelight beam to implement the aforementioned optical distance adjustment,as shown in FIG. 24D.

The beam-splitting optical element and the first and second opticalbeam-bending elements may be formed integrally or separately.

NINETEENTH EMBODIMENT

FIGS. 25A to 25C illustrate the structure of an optical displacementsensor to be used as an optical encoder according to the nineteenthembodiment of this invention.

The behavior of the principal axis of the light beam that travels from acoherent light source 10 to the first photosensor 3 is the same as thatof the thirteenth embodiment in the optical sense.

The following will therefore discuss only the difference between thenineteenth embodiment and the thirteenth embodiment.

FIG. 25A is a plan view showing the scale 2 in the nineteenth embodimentas viewed from the −z direction, FIG. 25B a cross-sectional view of thezy plane, and FIG. 25C a cross-sectional view of the xz plane.

The nineteenth embodiment of this invention is constructed as follows.

As shown in FIG. 25A, the scale pattern 21 is so formed as to have apitch p0 in the x direction.

In accordance with this difference, the light receiving areas are formedlike strips in cross section in the xz plane which corresponds to thecross section along c1-c1′ as shown in FIG. 25C, with the pitch beingp12 in association with the pitch of the second scale pattern, and thedeviation of groups of light receiving areas becomes p12/4.

The deviation p12 of the groups of light receiving areas isapproximately np11(z11+Z21)/Z11.

Though not illustrated, the cross-sectional structure in the xz planecorresponding to the cross section along c2-c2′ is such that a singlelight receiving area is formed on the second photosensor 3′ so that thelight beam that has been emitted from the light source 10 is caused todirectly enter the light receiving area on the second photosensor 3′ bythe beam-splitting means.

The operation of the nineteenth embodiment of this invention will now bediscussed.

First, the light beam that has left the coherent light source 10 has itsoptical axis bended by the diffraction grating 7 as the beam-splittingmeans as in the sixteenth to eighteenth embodiments, the resultant lightbeam then passes the scale 2 and the optical beam-bending elements againbefore reaching the first photosensor 3.

Then, the mode and operation for acquiring encoder signals from thefirst photosensor 3 are also the same as those of the sixteenth toeighteenth embodiments.

According to this embodiment, the light beam emitted from the coherentlight source 10 is reflected at, passes through or is deflected by thediffraction grating 77 as the beam-splitting optical means, and directlyenters the second photosensor 3′ where its intensity is detected.

The output of the second photosensor 3′ becomes proportional to the beamoutput.

When the beam output varies due to an environmental change or with thepassage of time, the output of the second photosensor 3′ is fed back tothe driving unit for the coherent light source 10 to suppress avariation in the output or the encoder output signals are compensated byusing the output of the second photosensor 3′, thereby always ensuringstable sensing of the displacement of the scale 2.

As this embodiment can detect the intensity of the light beam withoutusing the uniform reflectance or transmissivity pattern of the scale 2,it is possible to suppress an error in detection caused by a variationin the intensity of the light beam, reliably without depending on adefect, dust or stain.

Of course, the individual components of the nineteenth embodiment ofthis invention can be changed or modified in various forms in accordancewith the above-described thirteenth to eighteenth embodiments.

TWENTIETH EMBODIMENT

FIG. 26 illustrates, as a perspective view, the structure of an opticaldisplacement sensor to be used as an optical encoder according to thetwentieth embodiment of this invention

The scale 2 has two scale patterns 21 and 22, the first scale pattern 21so formed as to have a pitch p11 in the x direction while the secondscale pattern 22 is formed as a single pattern with the adequate widthfor detecting the reference point.

The surface emitting laser 10 which is used as the coherent light sourceis so designed as to emit two light beams from different coordinates inthe x direction and y direction as shown in FIG. 26.

The first light beam is irradiated on the first scale pattern 21,thereby forming a diffraction interference pattern on the lightreceiving surface of the first photosensor 3.

The second light beam is irradiated on the second scale pattern 22, andits reflected light is detected by the second photosensor 3.

The surface emitting laser light source 10 is arranged in such a way asto include the principal axis 5 of the light beam emitted from the lightsource 10 and to be tilted only within the plane perpendicular to boththe scale surface and the spatial period direction of the first scalepattern 21.

The operation of the twentieth embodiment of this invention will now bediscussed.

The first light beam emitted from the beam-emitting window 101 of thesurface emitting laser light source 10 passes through the first scalepattern 21, forming a diffraction pattern on the light-receiving surfaceof the first photosensor 3. The first photosensor 3 generates an encodersignal in the same way as in the first to nineteenth embodimentsdescribed above.

The second light beam emitted from the beam-emitting window 101 of thesurface emitting laser light source 10 is applied to the second scalepattern 22 and reflected therefrom. The light beam reflected from thesecond scale pattern 22 is applied to the light-receiving surface of thesecond photosensor 3′. The second photosensor 3′ generates an encodersignal in the same way as in the fifteenth to eighteenth embodimentsdescribed above.

In the present embodiment, the surface emitting laser light source 10 isused as a coherent light source. Any light beam can therefore be emittedfrom any position desired. The first light beam can be reliably applied,first to the first scale pattern 21 and then to the first photosensor 3.Similarly, the second light beam can be reliably applied, first to thesecond scale pattern 22 and then to the second photosensor 3′. Thus, thedegree of freedom of designing the sensor is greater than in thethirteenth to nineteenth embodiments.

In addition, it is unnecessary to install the beam-splitting means at aspecific position. This helps to reduce the cost of installing thebeam-branching means.

The present embodiment is advantageous in that no optical energy losstakes place in the beam-splitting means. The embodiment can thereforeoutput a signal having a high S/N ratio.

The components of the twentieth embodiment can, of course, be modifiedor changed in various ways.

As shown in FIG. 27, the second scale pattern 22 and the light receivingareas of the second photosensor 3′ are formed cyclically, so thatdetection of the absolute value of the displacement of the scale can beaccomplished by the vernier encoder as per the fourteenth or seventeenthembodiment.

Alternatively, an absolute encoder other than the vernier encoder may berealized by providing multiple sets of light beams, scale patterns andphotosensors.

When there is some need to adjust the optical distance between theposition of the light source and the scale or between the scale and thephotosensor, the optical distance adjusting means 50 may be inserted onthe optical axis of the light beam to implement optical distanceadjustment, as mentioned earlier.

Although the surface emitting laser is used as the light source that canoutput a plurality of light beams in this embodiment, the type of thelight source is not limited to this particular one as long as it canemit a plurality of coherent beams.

(Operation and Effect)

According to one aspect of the present invention, the beam size on thelight emitting surface of the surface emitting laser is 3 μm or larger(which was difficult to achieve using the edge emitting typesemiconductor laser that had been used in conventional encoders) so asto provide a beam with a small spread angle of less than 20°.

If the spreading of the light receiving areas is limited to the vicinityof the principal axis of the light beam from the light sourceaccordingly, it is possible to set the spread angle of the light beamcorresponding to the this narrow spreading of the light receiving areas.This can permit a diffraction interference pattern to be formed on thelight receiving areas by effectively using the light beam output fromthe light source.

This can provide an optical displacement sensor which can output anoutput signal with an excellent signal amplitude and excellent S/N ratioeven if the arrangement of the light source, the scale and the lightreceiving element is deviated from the optimal one.

According to another aspect of the present invention, a plurality oflight beams are irradiated on different areas on the scale, therebyforming a plurality of diffraction interference patterns on differentareas on the light receiving surface.

A plurality of light intensity detecting means formed on the photosensordetect the light intensities of specific spatial-phase portions of therespective diffraction interference patterns.

This can ensure reliable isolation of signals from the individual lightintensity detecting means as compare with the case where the lightintensity detecting means for detecting the light intensities of aplurality of specific spatial-phase portions on the area of a singlediffraction interference pattern generated by a single beam.

According to another aspect of the present invention, a plurality oflight beams are irradiated on different areas on the scale, therebyforming a plurality of diffraction interference patterns on differentareas on the light receiving surface, thus ensuring reliable isolationof signals from the individual light intensity detecting means.

According to the fifth embodiment, the scale has a common diffractionpattern, so that for the individual detecting means to detect the lightintensities of specific spatial phase portions of the respectivediffraction interference patterns, the light receiving areas thatconstitute each detecting means should be arranged in a predeterminedpositional relationship. This makes it inevitable to prepare the lightreception patterns again when the pitch of the scale differs.

When there are a plurality of different diffraction interferencepatterns, by way of contrast, the difference in phase among a pluralityof diffraction interference patterns detected by the individualdetecting means can be set based merely on the positional relationshipamong the diffraction interference patterns on the scale. Thiseliminates the need for changing the arrangement of the light receivingarea constituting each detecting means.

According to another aspect of the present invention the scale surfaceand the light receiving surface of the photosensor are tilted to theprincipal axis of the light beam which is emitted from the coherent(laser) light source. It is thus possible to prevent the light beam fromthe laser light source and reflected at the scale or the surface of thephotosensor from returning to the light source, thereby suppressing thesuperimposition of noise caused by the returning laser light on theoutput signal of the sensor.

This embodiment can therefore ensure scale displacement sensing athigher precision and higher reliability.

According to another aspect of the present invention, the pitchdirection of the diffraction grating is arranged in parallel to-thelight receiving surface of the photosensor, the spatial period of thediffraction interference pattern on the light receiving surface becomesconstant, which simplifies the pattern design and layout of the lightreceiving areas on the photosensor.

With the structure that does not specify the pitch direction of thediffraction grating formed on the scale and the tilt direction of thelight beam which goes out from the light source, in general, when thedistance between the scale and the light source changes, the diffractioninterference pattern on the light receiving surface is shifted in thepitch direction of the interference pattern. It is thus difficult todistinguish this movement from the movement of the diffractioninterference pattern on the light receiving surface which is caused bythe displacement of the scale in the pitch direction of the diffractiongrating on the scale.

By arranging the principal axis of the light beam perpendicular to thepitch direction of the diffraction grating, however, the diffractioninterference pattern on the light receiving surface is generatedsymmetrically to the principal axis of the light beam. Even if thedistance between the scale and the light source changes, therefore, thediffraction interference pattern on the light receiving surface does notmove in the pitch direction on the principal axis of the light beam.

If a plurality of light receiving areas are formed only in the vicinityof the principal axis of the light beam, particularly, the sensoroutputs from those light receiving areas near the principal axis of thelight beam are scarcely affected by a change in the distance between thescale and the light source. This makes it possible to accurately detectthe displacement of the scale in the pitch direction of the diffractiongrating.

It is thus desirable that the surface emitting laser which has a beamspread width corresponding to the distribution of the light receivingareas in the proximity of the principal axis.

The structure which has the light source and the photosensor arranged onthe same side with respect to the scale will hereinafter be referred toas a reflection type structure.

According to another aspect of the present invention, the light sourceand photosensor can be designed compact and integrated, so that thesensor head can be made smaller as compared with the structure which hasthe scale sandwiched between the light source and photosensor(hereinafter referred to as a transmission type structure).

In the case of the reflection type structure, when the distance betweenthe scale and light source changes, z1 becomes z1+Δz and z2 becomesz2−Δz.

Let us now consider the case where the scale surface is arranged inparallel to the light receiving surface.

Suppose that the pitch of the interference pattern to be formed on thelight receiving surface changes to p2′ from p2 when the positionaldeviation of Δz has occurred. Then, the equation (4) is satisfied forthe reflection type structure and the equation (3) for the transmissiontype structure.

When the aforementioned tilt angle is small, therefore, the reflectiontype has a smaller difference between the p2 and p2′ for the same Δz.

That is, even if the distance between the scale and the light source isshifted, the reflection type structure has advantageously a smallerdeviation in the pitch of the diffraction interference pattern.

With z1=z2 under the condition that the equation (1) is met, inparticular, the following equation is satisfied, the reflection type hassuch an advantage that p2 is not affected by Δz.p2′=p2=2p1   (5)

According to another aspect of the present invention, the pitchdirection of the diffraction grating is arranged in parallel to thelight receiving surface of the photosensor, so that the spatial periodof the diffraction interference pattern on the light receiving surfaceis constant, which simplifies the pattern design and layout of the lightreceiving areas on the photosensor.

With the structure that does not specify the pitch direction of thediffraction grating formed on the scale and the tilt direction of thelight beam which goes out from the light source, in general, when thedistance between the scale and the light source changes, the diffractioninterference pattern on the light receiving surface is shifted in thepitch direction of the interference pattern. It is thus difficult todistinguish this movement from the movement of the diffractioninterference pattern on the light receiving surface which is caused bythe displacement of the scale in the pitch direction of the diffractiongrating on the scale.

If the pitch direction of the diffraction grating is made perpendicularto the principal axis of the light beam emitted from the aforementionedcoherent light source, however, the diffraction interference pattern isnot shifted on the light receiving surface in the pitch direction in thevicinity of the principal axis even when the distance between the scaleand light source changes for the same reason given in the case of theeighth embodiment. This makes it possible to accurately detect thedisplacement of the scale in the pitch direction of the diffractiongrating.

It is thus desirable that the surface emitting laser which has a beamspread width corresponding to the distribution of the light receivingareas in the proximity of the principal axis.

Even if the distance between the scale and light source is changed, thisreflection type structure has an advantage that a change in the pitch ofthe diffraction interference pattern is small.

According to another aspect of the present invention, the light beam maybe irradiated to be perpendicular to the surface of the scale. In thiscase, it should be noted that return light noise is generated by thelight that returns to the light source from the scale or the lightreceiving surface when z1 is small. In the usage where such noisegeneration is negligible, however, this embodiment has such advantagesthat even if the distance between the scale and the light source ischanged, a change in the pitch of the diffraction interference patternis smaller, and the positional deviation of the peak of the diffractioninterference pattern is smaller, as compared with the transmission typestructure used in the prior art.

Even if z1 is small, the noise that is caused by the light returning tothe light source from the scale or the light receiving surface isreduced by subjecting the top and bottom surfaces of the scale or thelight receiving surface to an optical process of reducing thereflection, this structure can be adapted for any use where the returnlight noise is negligible.

According to the present invention, the use of the surface emittinglaser light source can make the spreading of the light beam smaller andcan provide an output signal with an excellent S/N ratio even if thearrangement of the light source, the scale and the light receivingelement is shifted from the optimal one.

A beam-splitting optical element may be disposed in such a way as toinclude the principal axis of the light beam immediately after it hasbeen emitted from the coherent light source and to split the principalaxis of the light beam into a plurality of directions only in the planeperpendicular to the pitch direction of the first scale pattern. Even ifthe spatial gap between the scale and the light source is changed,therefore, the diffraction interference pattern on the light receivingsurface is not shifted to the pitch direction of the scale because ofthe principle that has been explained earlier with reference to FIG.17A.

This can ensure both the advantage such that an error in positionaldetection hardly occurs even when the spatial gap between the scale andthe light source is changed, and the additional provision of thefunction to monitor the intensity of the optical output of the coherentlight source, the absolute position detecting function, the detectingfunction based on the reference point pattern, and the like.

The second light beam that has been split by the beam-splitting opticalelement is irradiated on the second scale pattern, which may have auniform reflectance, transmissivity or diffraction efficiency.

As the intensity of the light beam that has been reflected at, haspassed through or has been diffracted by the second scale pattern isdetected by the second photosensor, the function to monitor the opticaloutput of the coherent light source can be realized.

Even if the ambient environment of the sensor is changed, the opticaloutput can be stabilized by feeding the output of the light intensitydetecting means comprised of the second photosensor back to the drivemeans for the laser light source, thus ensuring stable sensing withrespect to such a status change.

According to the present invention the number of the tracks of the scalepattern may be increased to increase the number of associated, splitlight sources and the number of associated photosensors.

The second scale pattern may alternatively have a predetermined periodp12 different from that of the first scale pattern, thereby generating adiffraction interference pattern having a spatial periodp2=p12(z11+z21)/z11 on the light receiving surface on the secondphotosensor.

As light receiving areas with a spatial period p22=np2 are formed on thesecond photosensor in the x direction, the second photosensor outputs aperiodic signal intensity every time the scale moves by p12 in the pitchdirection (x direction) of the scale pattern.

Therefore, the intensities of the outputs of the first and secondphotosensors respectively vary at periods of p11 and p12, the amounts ofdisplacement in the pitch direction of the scale. This can permitaddition of the absolute position detecting function within the range ofthe scale movement which is the least common multiple of p1 and p12based on the principle of vernier encoder.

The second scale pattern may be a single scale pattern or a plurality ofscale patterns formed at a predetermined reference position.

The intensity of the light beam that has been reflected at or has passedthrough the second scale pattern is detected by the second photosensor.

Because the output of the second photosensor changes only when thesecond light beam is irradiated on the second scale pattern at thereference point, it is possible to add the function of detecting thereference point with respect to the movement of the scale in the pitchdirection.

In the embodiments shown in FIG. 20B and FIG. 24D, all of thebeam-splitting optical elements are arranged in such a way as to includethe principal axis of the light beam immediately after it has beenemitted from the coherent light source and to split the principal axisof the light beam into a plurality of directions only in the planeperpendicular to the pitch direction of the first scale pattern. Even ifthe spatial gap between the scale and the light source is changed,therefore, the diffraction interference pattern on the light receivingsurface is not shifted to the pitch direction of the scale because ofthe principle that has been explained earlier with reference to FIG.17A.

The second light beam that has been split by the beam-splitting opticalelement and irradiated on the second scale pattern having a uniformreflectance, transmissivity or diffraction efficiency, may then beirradiated on a third beam-splitting optical element where its principalaxis is deflected again so that the light beam is led to the secondphotosensor.

As the intensity of the light beam that has been reflected at or haspassed through the second scale pattern and has further been deflectedby the third beam-splitting optical element is detected by the secondphotosensor, the function to monitor the optical output of the coherentlight source can be realized.

Even if the ambient environment of the sensor is changed, the opticaloutput can be stabilized by feeding the output of the light intensitydetecting means comprised of the second photosensor back to the drivemeans for the laser light source, thus ensuring stable sensing withrespect to such a status change.

This aspect of the present invention corresponds to the seventeenthembodiment of this invention.

The embodiment also covers the structure where the number of light beamssplit and the number of photosensors are increased in accordance withthe number of the tracks of the scale pattern.

In the embodiment shown in FIGS. 22A-22C and FIG. 24B, the second lightbeam split by the beam-splitting optical element is irradiated on thesecond scale pattern of the predetermined period p12 different from thatof the first scale pattern, and is then bended by the first opticalbeam-bending optical element, thereby generating a diffractioninterference pattern having a spatial period p2=p12(z11+z21)/z11 on thelight receiving surface on the second photosensor.

According to another aspect of the present invention, the first lightbeam emitted from the coherent light source is irradiated on the firstscale pattern, thus forming a diffraction interference pattern having aspatial period p2=p11(z11+z21)/z11 on the light receiving surface of thefirst photosensor.

When the scale is displaced by p11 in the pitch direction of thediffraction grating, the diffraction interference pattern is shifted byx2=p11(z11+z21)/z11 on the light receiving surface in the samedirection. Therefore, the first photosensor provides an output signalwhich changes with a periodic intensity every time the scale isdisplaced by p11 in the pitch direction of the diffraction grating.

The second light beam is irradiated on the second scale pattern and itsintensity is detected by the second photosensor which receives the lightbeam that has been reflected or diffracted by or has passed through thesecond scale pattern.

By adequately setting the first and second scale patterns, it ispossible to achieve the function of monitoring the intensity of theoptical output of the coherent light source by detecting the intensityof the light that has been reflected or has passed through the scalehaving a constant reflectance or transmissivity, the absolute positiondetecting function by a vernier encoder, the function of detecting theorigin based on the reference point pattern, and the like.

Further, two or more of those additional functions can be achievedsimultaneously by increasing the number of light beams split and thenumber of photosensors in accordance with the number of the tracks ofthe scale pattern.

Although all of the coherent light source, the scale surface and thelight receiving surface cannot be arranged in parallel to one another,this structure has such an advantage as to eliminate the need for thebeam-splitting optical element and a step of assembling it.

(Effect of the Invention)

As discussed above, this invention can provide an optical displacementsensor, which permits the spread angle of the light beam to be set at anangle not larger than a predetermined small value that cannot beachieved in the conventional semiconductor laser light sources, therebyproviding an output signal of a good S/N ratio even when the arrangementof the light source, the scale and the light receiving element isdeviated from the optimum one.

As apparent from the foregoing description, this invention can alsoprovide an optical displacement sensor capable of preventing the lightbeam from returning to the light source, thereby suppressing thesuperimposition of noise caused by the returning laser light on theoutput signal of the sensor.

Furthermore, this invention can provide an optical displacement sensorwhich reduces changes in the period and position of the diffractioninterference pattern on the light receiving surface so as to suppressreduction in the signal amplitude and a change in the period withrespect to the scale displacement, even when the arrangement of thelight source, the scale and the light receiving element is deviated fromthe designed arrangement.

Moreover, this invention can provide an optical encoder which can beused as an optical displacement sensor, has the reference pointdetecting function and the absolute point detecting function so as to beable to accurately detect the displacement of the scale in the xdirection while being hardly affected by the change in the gap betweenthe scale and the head, and has a structure and means for reducing theassembling cost and for stabilizing the encoder signal output regardlessof a change in the ambient environment by employing a mounting mode thatdoes not use an inclined substrate.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An optical displacement sensor comprising: a coherent light sourcewhich emits a light beam; a scale which is movable with respect to thecoherent light source along a relative movement direction, and whichincludes a grating pattern formed on a pattern-formed surface thereof tohave a predetermined period in the relative movement direction, whereinthe light beam is emitted by the coherent light source to thepattern-formed surface along a path which is substantially perpendicularto the pattern-formed surface; and a photodetector having a lightreceiving surface which detects movement of a diffraction image of thelight beam reflected and diffracted by the grating pattern, said lightreceiving surface including a plurality of sets of light receivingareas, wherein the light receiving areas in the sets are arranged atintervals of p2 in the relative movement direction, and the plurality ofsets of light receiving areas are shifted with respect to each other inthe relative movement direction by an odd multiple of ¼×p2; whereinp2=np1(z1+z2)/z1where: n is a natural number; p1 is a spatial period ofthe grating pattern on the scale; z1 is a distance along a principalaxis of the light beam from a beam emitting surface of the coherentlight source to the pattern-formed surface of the scale; and z2 is adistance from the light-receiving surface of the photodetector to thepattern-formed surface of the scale; wherein the coherent light sourceand the photodetector are arranged on a same side of the scale, and thelight receiving surface of the photodetector is substantially parallelto the pattern-formed surface of the scale; and wherein relativemovement of the scale with respect to the coherent light source isdetected based on an output of the photodetector.
 2. The opticaldisplacement sensor according to claim 1, wherein the coherent lightsource comprises a surface emitting laser.
 3. The optical displacementsensor according to claim 2, wherein at least one of the scale surfaceand the light receiving surface of the photodetector comprises areflection reduction structure.
 4. The optical displacement sensoraccording to claim 3, wherein the reflection reduction structurecomprises one of a reflection preventing film and a surface scatteringstructure.
 5. The optical displacement sensor according to claim 1,wherein the plurality of sets of light receiving areas comprise foursaid sets of light receiving areas, and the photodetector furthercomprises an output terminal form monitoring sums of outputs of the foursets of light receiving areas.